Really it has to do when the birthday cut off is for the sport. It differs for baseball, hockey, and soccer.

The relative age effect is documented in a wide range of sports and also in academics. A child born in January will be 11 months older than a child born in December the same calendar year, which is a big relative difference at young ages. If coaches don't think of this, they will misperceive the older kid as being more mature and talented, and will then encourage the older kid more.

I took data on all players in the FIFA World Cup and checked their birth month. Just as expected, the most common months to be born are January, February and March.

Some countries may have other cutoff dates for starting school or sports teams than January 1, but despite this, the effect shines through.

Made in R using the ggplot2 package. Data from fbref.com.

Yes, and between countries. But I thought I would just do something simple here (most countries probably use January 1 as the cutoff), and I think the fact that it shows up despite the coarse analysis shows the importance of the phenomenon.

I assume that September spike is the places in eg Europe that group on school year for youth games?

I was born in March and am still shitty at everything.

Can be! Or randomness.

September also is a months in which many births occur in Europe because 9 month earlier is December when people spend much time with their partners and indoor activities are more popular than outdoor activities.

It might be more complicated but I’d be curious to see this data adjusted to ‘month of the school year’ rather than calendar year.

IE in the UK, a September birthday would be listed as a ‘month 1’ as it’s the first month in our school calendar, and how age for this sort of thing is calculated. I think varying school start dates by country explains a few of these random peaks, but it’s hard to be sure.

Thank god something I can show my fiancée. She really doesn’t understand it. I by no means need to have my next child in January but I just won’t plan to have a late year baby.. for girls it’s not much of an issue, but it makes a difference for boys early in life and sports.

My fiancee really doesn’t understand why I want to plan to have our child earlier in the year. I seen the effects first hand playing competitive sports at a young age.

For academics September is the relative age effect start point. Most sports in Northern Europe work the same way Football 100% does so here in the UK

My twin brother and I repeated 1st grade and I always wonder if that was responsible for our academic success. Always being a year older is a huge advantage when you're a kid. Definitely helped in sports too because I hit puberty a grade early and was like 6in taller than my peers in middle school.

Well good news is if you are perusing Reddit and not busy doing something else then your child most likely won’t advance far into competitive sports so feel free to have it whenever

So you just woke up and chose violence? Perhaps one of the most useless comments I have ever read, thanks for the invaluable input.

This is sad.

Sad to want to give your child the best possible life with as many advantages as you can provide?

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It is what it is the truth hurts us all my friend

It depends on the country. In Sweden where I live January is the cutoff for both schools and most sports.

Having a semi-professional sport background and a cushy desk job that allows me to surf the web definitely indicates a child that will be unable to perform well in sports you are right.

I was born in the beginning of february and my parents sent me to school a year early. Good job parents.

This would make sense in the Southern Hemisphere where school is February to December, but makes less sense in most of the world, where school begins in August or September.

Benford’s Law

I immediately thought of Benford’s law Considering the months as their numbers 1, 2, 3..

“A logarithmic scale bar. Picking a random x position uniformly on this number line, roughly 30% of the time the first digit of the number will be 1. A set of numbers is said to satisfy Benford's law if the leading digit d (d ∈ {1, ..., 9}) occurs with probability[10]”

A really cool episode of Connected: The Hidden Science of Everything on Netflix, explains the law really well, and has IRS help explain (elusively) how it’s used to detect fraud in real life.

This should not be done by calendar month but rather relative to school year start depending on their country. A September child in a September school year is relatively the same age as a January child in a January school year.

And that’s why your child won’t make it far, most athletes are dictated by genetics and discipline. If we go by chance and your Reddit history we can assume your offspring won’t make it far

England, Wales, Belgium, France, Germany, Croatia, Netherlands, Poland, Portugal, Serbia, Spain and Switzerland all start in September (academics)

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Many people are suggesting you should take into account the country of the player, but I'd say to take into account the BIRTH country of the player. Many players are "imported" from other countries.

Benford's law doesn't apply here. It is (best) for numerical values that is spanning multiple orders of magnitude. But instead we have a fixed set of values (Jan-Dec) with no positional notation. If we were to convert the month to their numerical values and try to apply Benford' law, we would need to group the numbers for January, October, November and December to the groups of values starting with 1. That would be a bit more "benfordy", but really overshoot in other direction.

It depends too much on the organization to really be able to split it up for every case.

In the US age /birthday cutoffs vary place to place, but Aug-sept is most common for school classes meaning you are right a child born in September is 10-11 months older than some of their peers in the same class which can be a significant advantage particularly in early years.

Kids' sports (and also many other non sport related extra-circular activities not directly sanctioned by the school) tend to be broken down either by grade, in which case the ~Fall-Late Summer range is most relevant, or by age regardless of grade, in which case Jan-Dec will be the range that matters.

Same in Canada.

Ahhhhh totally makes sense. Thank you for deepening my understanding of this cool phenomenon.

Interesting article breaking down just North American sport leagues.

https://www.researchgate.net/publication/290649143_Does_Relative_Age_Affect_Career_Length_in_North_American_Professional_Sports/download

It's interesting, and it kind of works to break it down this way. I didn't fully articulate what my concern is in my other comment, my question is how do you account for situations in which the same person in the same activity would be on opposite ends of the age gap depending on where they play.

A child born late in the year would have the most advantage in school sports, even say just PE class all the way up to the varsity team in high school, but that same individual (particularly as a younger child) would be on the bottom end of the age gap advantage in a city sponsored league that uses the calendar year

Well, academic year in Poland starts in september but you still go to classes according with year you were born in. Children from January wil be the oldest in class and the ones from December the youngest(There are also cases where a children from example December can be moved to another year)

Here's the data on hockey. https://www.quanthockey.com/nhl/birth-month-totals/nhl-players-career-stats.html

That is not correct, at least in Spain. The classes start on September, but the students go to class following the calendar year. Someone born in April and someone born in November of the same year will go to the same class. And the same for sports.

Interesting data, but shouldn’t this also be compared to the average birth month pattern in general? That would provide more evidence for the relative age effect. Births are not equal across all months to start with in humans (even simplistically based on days of the moths).

Doesn't matter when schools starts, it matters where the age cutoff is. I live in Canada where school starts in September, but you are divided by birth year. Is that not the norm in the Western World?

It's probably just the fact that September is the most common birth month

In the US (well at least my school district in Minneapolis) it oriented around a school calendar i.e. those born September 2000 to August 2001 were all in the same class. People born in September and October were always the oldest in their class. Long story short this graph and the interpretation of this graph is wrong for a number of geographical and cultural inconsistencies.

This was covered by Malcolm Gladwell in Outliers, except for hockey.

This is true and I suspect that would explain the spike in September!

Or it could be because they're generally the biggest, smartest and most mature of their peer group they develop better confidence and leadership skills and are therefore more likely to pursue things to their highest level.

Or maybe a little from both.

I often wonder about this - how birth month might affect academic or sporting performance. Someone must have done this research to some extent, surely? I'm from the UK, had kids in Mexico (where school year cutoff was December at least in our district) and now live in US, where it's October, but where so many parents try to game the system by holding their kids back that some districts are really strict on age/school start dates. It's fascinating.

It basically gets lower and lower throughout the months

Much of the US goes by the Sept cutoff too. There are some places that use raw age for sports but most use school class and the cutoff varies but usually is around Sept 1st or Labor Day

My countries national football federation (KNVB🇳🇱) did a study about this and it was in fact right. Many younger players good players will never make it to the professional stage because they are physically underdeveloped in the junior categories.

Sadly, most professional clubs focus on prestation instead of potential.

Same can be observed in all major European pro leagues

The parents did coitus in the preceding April, May and June, or the parents eloped to prepare for the World Cup in June/July and then stopped once competition began.

Since the outcome of the relative age effect is largely due to ,perceived’ talent when compared to peers and therefore better support, you could technically overcome the relative age dilemma by simply encouraging your kid regardless of what month they were born in :)

The September one can be explained because some countries have a September cutoff for academics/sports instead of January. At least, from my experience, UK and US have a September cutoff. In Spain, France, Italy Im sure the cutoff is in January, I’m not sure about the rest.

That would explain the September jump, I would guess that one or more countries also uses June as the cutoff

It’s the cutoff, not when it starts. In most European countries such as my own, you play with people from your age group, which is your calendar year. So, people born in January are playing with people born in December, nearly a year younger. This is especially beneficial for physical sports where a year difference can be very influential.

But it's about coaches and teachers, not the parents.

There are two cutoff points, one in September at the beginning of the school year, and one in January at the beginning of the calendar year.

Oh hey guess where the two spikes are...

Correlation is not causality.

Nope, that's all joke.

Kids born at the beginning of the year used to be bigger than ones from the end of the year, and that's why have better chances to make a professional career, as teams need bigger players, rather than extremely talented ones.

This sounds like bullshit. Where on earth is age measured in the birth year instead of the birth day?

Yes, great book. I thought he covered hockey as well.

Outliers by Malcom Gladwell is basically about this and has a section on how kids born at a certain time of year have advantages over others (in sport), as they're more physically developed, and are more likely to getting scouted. Highly recommend if this kinda thing interests you.

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Need to normalize by total number of people born in each month, I assume it’s not uniform

9 months before September is December, most people take a lot of December off because of the holidays. It's also the end of the year when people reflect on their lives and what they may want to do next so I think it makes sense that a lot of people try for a baby around this time of year.

This just backs up what I've been saying for YEARS about children's sports.

It has NOTHING to do with talent, and EVERYTHING to do with maturity.

Your kid isn't "good" at sports, they're just bigger than most people they're playing against.

If you mature early (like I did), you will likely be "good" at sports. (I was 5'10" in 8th grade.) You will be pushed to do sports more than someone who matures later, so, by the time that talent DOES come into play, you have many many more hours of training than someone who didn't start playing until a few years later.

It's relatively rare that someone matures late or is small and their talent alone carries them to the furthest levels of sports. (Think of people who were the "wrong" size for their sport. Generally someone who is very small in most american sports.)

Malcolm Gladwell wrote about this phenomenon in his book "Outliers"... great read.

He’s done an update on his podcast recently. Now some parents are using his data to intentionally hold their Children back a year to give them an academic advantage. Enough to start skewing the data.

Edit - I don’t think gladwell came up with this idea but he did make it more mainstream.

Malcom gladwell did an update on his podcast and observed that many parents are using his data as a reason to hold their children back a year to give than an academic advantage.

Was just going to say this!

It would be easy to enough to adjust for birth year. Instead of just birth month, it would be number of months older than the minimum.

For instance, someone who turns 17 on Sep 1 of their senior year (12th grade) would be considered "0 months" old. The typical range would be 0-12 months if everyone was born in that year, but someone who was held back and turned 18 two months before their senior year started would be "14 months". Your range might be as much as 0-24 instead of 0-12.

As a economist, was going to say that. This is partly inconclusive data. It’s need to be crossed like you said with average births per month maybe across healthy males between 8-18 years of age and 18-35, to differentiate if this affects only in youth or in adulthood as well

Divide the data by hemisphere. It would be really interesting.

Yep... I loved that chapter. If the sports leagues are really interested in finding the best players regardless of birthdates, they could do a sliding cutoff that changes 3-6 months every year.

That way the "January" kids would sometimes be the oldest in their group, and would sometimes be the youngest. Depending on how they did it, a kid might either spend two seasons in one age group, or skip an age group altogether.

I listened to it a while ago but that sounds similar to what he ended up doing.

I think he means "Gladwell discussed this phenomenon, except the sport was hockey instead of soccer." Outliers specifically discussed Canadian youth hockey.

Probably need to separate them into northern and southern hemisphere players

And also, error bars and statistical significance tests would be useful here

I feel like the kind of parents who plan January births so their kids can have some tiny advantage against their classmates are the kind of helicopter parents that are going to push their kids at every stage and the kids may wind up more successful and in therapy.

Outliers by Malcolm gladwell has a good explanation for this

The book "Outliers" explains exactly why this phenomenon happens.

As well as the school cut off from kindergarten.

The youngest kids are more likely to underachieve. Amazing that it's so clearly pronounced though.

Or it just explains that May is popular mating season ;)

Would be even better if could be corrected for relative worldwide birth rates per month. For example, September appears to spike, but I recall births being higher in the northern hemisphere in September

My friends planned this, they planned to have a baby in January for the same reason. They ended up with a premature baby’s who was born in December.

I mean it can happen. I wouldn’t necessarily aim for a child to be born in January for this reason. My son now was born in April. And If I could “aim” for a date it would be Feb/March. People can deny it all they want but I’ve seen too many late year baby boys get bullied for being smaller - even if it’s just In the early years of school that shit leaves a lasting negative affect.

There is an excellent Veritasium (YouTube scientist) video that goes into this:

https://www.veritasium.com/videos/2020/8/28/is-success-luck-or-hard-work

Highly recommended watch.

No shade from me. I won’t fault any parents for wanting the best for their child. I had fertility issues so I am just happy I had a kid whenever it happened, I couldn’t plan it like that. Tho I have also seen many kids born later on the h the year who were big and strong (I am a teacher), so many factors go into how a child becomes (genetics, diet)

As a Brit, my first thought was "wow that's a weird way of doing it" then my second thought was "Hang on, no it isnt. Its the only logical way of doing it. Why the fuck do we still start a new school year in September???".

Idk which specific sport you're concerned with and if it would be done through school or separately, but if we're going with school grade cutoffs then why not just let the dice fall and if they are near the cutoff then keep them home for an extra year? They would be the oldest in their grade, but only by the same margin that the the otherwise oldest kids are/would be.

Sports here (Canada) don't go by school grade... they go by birth year. Sports aside, I would rather not have a December 2024 baby in classes with 2025 kids. I'd rather my kid be a Jan-March 2025 kid.

BMX just uses your current age to set cohorts. If you're 8 and it's your birthday later this week, you race the 8 year olds since you're still 8.

Next week, you'll be 9, so then you'll race against the other 9 year olds.

There's still an effect - since at least here in Canada the sport shuts down for the winter, so people born late fall would spend the most race days in the cohort as an older athlete, but I'd imagine it's muted vs other sports using a calendar approach.

Though with an individual sport like BMX, it's easier to do this since it doesn't have the same team forming process . You just show up at races and race whoever happens to also show up.

Well we start the new school year in august, but the cut-off is still January! Everybody that is born a specific calendar year starts school the same time in August. If you are born December 31 the year before you start in August a year earlier.

Oh, well then my first thought was correct. Your way is weird! At least out cutoff is start of september as well.

Glad that worked out for you, being a parent is a wonderful thing. (My first is 7 months old). Genetics will always play a major part but that I can’t control (not some gene editing nut job) but if you have genetics as a December baby to outgrow and outperform everyone else your age, just imagine that same kid as a January baby. It’s really just a preference thing and it’s a small thing I can control to help tip the odds in my child’s favour in this difficult world.

I always felt he missed talking about the few kids that actually made it from the last months. What made them succeed in spite of being in that cohort? Weren’t they the true outliers?

This is a big known issue in football. They've been working to remedy it in England this century as previously the overwhelming majority of players in the premiership were born in the second half the year, that meaning they were oldest in their school year.

I know several countries have the normal year for their school year cut offs which could be shown here.

Yes. Big historic issue in England that your chances of being scouted are much higher if you're born in the second half of the year so are the biggest in your age group.

These days scouts actively seek to look beyond this. Trying to look at fundamental ability rather than heavily physique related performance.

Ahh, that works. And you're right that it's much easier in a solo sport than a team sport. It'd be tough on a coach if every time a kid had a birthday, he moved on to the next team.

This and the 10,000 hour rule which is a result of this is one of the main thesis of Gladwell’s Outliers.

Not in Scotland. The oldest kids in the year are born 1st March, the youngest end of Feb. The September kids are squarely in the middle.

I live in Argentina where school year is March- December and you're divided from July (born in year n) to June (born in year n+1)

Trying to use some psuedo-scientific garbage to prove that this isn't the Age of Aquarius amd we don't run the world! Fer shame!

In India, reporting the wrong birth month to the school is super common (or it used to be), from parents trying to optimize their kids' chance of academic success.

Why is april so low, its in the middle of spring.

September is 9 months from december… They fuck a lot on the holidays

The theory is that if you're older each year in your youth playing with and against younger players you don't improve skills as much, and if you're younger playing with and against older players you can get better.

There's different cutoffs for different sports at different times of year based on eligibility and when each sport plays in the year, so different birth months have different effects on different sports.

That's my understanding at least, but not particularly answering your question about April.

Exactly and then track the delta and see if it's significant

We figured out on my high school varsity team that ~60% of the team was born between September and November. We then realized that because of the cut off for the grade being September, those born in the early months of the school year then got to play with the older kids who were a grade level above them, making them better players.

In America, at least in my school system in California, the cutoff is in November. So a person born December 2003 - November 2004 would have graduated high school in 2022, someone born December 2004 - November 2005 will graduate high school in 2023, etc. There are some exceptions on the older side, children who started school a little bit late.

At my high school, which I graduated from in 2022, there is a student who was born in October 2005 and is currently a junior. Another student born in November 2005 is currently a senior. There are a few similar cases in other classes.

It's actually the opposite. January and February are the highest above and for hockey, because these kids are usually more physically developed than peers. This leads to being selected for elite camps/teams earlier, which compounds throughout their lives. Another cool economics type phenomenon in soccer (may no longer be true), a study was done early 2000s, there's a perfect balance of wealth of parents on average for professional soccer players in Brazol: enough that their nutrition is never sacrificed, but not enough money for structured programs after school, means they're fit but also non stop playing street soccer.

US has september cutoff as well.

August kids are the youngest in the class and September kids are the oldest in the class.

OP's image and title are directly about this...

That's crazy! I could totally see that happening some places here too though.

So do parents in India want their children to be the oldest in class?

i thought there was a bigger divide actually, this looks less extreme than i would expect, are different countries perhaps using different date cutoffs? i'd want to see it by country for that reason

someone posted hockey stats and i thought i'd seen them in the past and saw a more extreme divide as well, it's a lot less than i remember

except in Germany for example we don't group football teams the same way we do with the school year. for football it is an entire year from Jan to Dec, for school it is from July 1st to June 30th

And also by country

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Or you could do what they do in the southern USA and have them early in year AND hold them back a year. I started college at 17 and my roommate turned 20 freshman year.

It also doesn't help if you're late going into puberty. I'm a November and never hit puberty until late in my 14th year. I was smaller in the first part of 10th grade than some of the kids entering 8th grade. It's hard to gain any skills in sports if you're never played because of your size.

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https://i.imgur.com/ML9zrvi.png

so i ran a quick calculation on this website: https://www.graphpad.com/quickcalcs/grubbs2/ thats an outlier test and according to the result the 94 isn't an outstanding value, i'd assume then its just a random effect

The book "Outliers" by Malcolm Gladwell dives into this phenomenon. Great read.

He didn't, in the podcast he talks about the Canadian researcher's wife who noticed it.

That's what the relative age effect is.

Yes, according to my co-workers from India. They're pretty upfront about it, too (and pretty upfront about being in arranged marriages).

Come to think of it, I don't remember providing a copy of the birth certificate when I enrolled my kid in kindergarten. I think most American parents are so eager to get their kids out of the house and into free childcare (er, I mean school) that the deception would be more likely to go the other way (pretending your kid is older than they really are).

In my neck of the woods (California) the cutoff is September 1, but it used to be December 31. You had a lot of kids going halfway through Kindergarten as 4-year-olds. Now those Sept-Dec babies go to "transitional kindergarten" first.

As the other person commented, it’s the opposite. At young ages, the difference of 11 months is a huge percentage.

Let’s say we have 2 players we have to choose between. Player A and Player B. Both were born in the same calendar year, so for many sports they are grouped together for divisions and such.

Player A was born in January

Player B in December.

Let’s also imagine that they are 8 years old, trying out for their first travel team. At this point in their lives, 11 months of extra growth has given Player A a slight advantage, and the coach picks him for the team. Player B doesn’t make it, but sticks with the sport in recreational leagues.

Player A however gets more specialized coaching, more practice since they’re also playing rec league and travel, and they get more experience playing at a more “serious” level.

Well now it’s a year later, and time for tryouts again.

Player A still has 11 months of growth over Player B, but he’s also got a year of more intensive, in depth training, and more time playing the sport. Even though Player B has been practicing his best, and playing rec level, he just hasn’t developed at the same rate. Plus, he’s still 11 months behind on his physical growth. This gap in skill, experience, and maturity has prevented him from making the team again.

A year later, their 3rd tryouts, will show the divide between the two players has grown even further than the previous years, and any hope of Player B catching up is looking unlikely without some serious, specialized training.

This cascading effect continues throughout their lives, and the effects can be seen greatly when puberty enters the equation, and the rapid development of some players, but not others, coincides with important milestones, like making a high school sports team, or some other big event.

Sports organizations that want to categorize which age divisions players play in. They don’t care about what day, they want your birth year, and to group you with other kids of the same year.

I was definitely one of those parents desperate to get my kids to school as soon as possible! But one has a September birthday and is the youngest in his year, and I know that other parents, in a different position (I've, who could afford private childcare) held back their kids with September birthdays because of this idea they would be more likely to get college sports scholarships or do better or whatever. Not being from the US and having gone to uni with people from other countries (who start school at different times and ages), i have always thought this is really weird -- like surely, and compared to kids from other nations with different school systems, surely this can't still be giving intentionally-held-back kids an edge once they are older?

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I don't mean that everywhere in Europe does it, but quite a few places do, some of which are in europe (eg the UK)

Wouldn’t the age effect be different for countries where the school year starts in Jan-Feb instead of September?

>surely this can't still be giving intentionally-held-back kids an edge once they are older?

That's the weird thing: It does. Being the oldest/tallest/strongest/smartest kid in your cohort means coaches give you more game time, or you gain more academic confidence and skill among your peers in early grades. And it snowballs from there.

I just find it hard to believe that that applies universally across countries and cultures and hemispheres and school systems and everything. But maybe I just don't want to believe it because it means I already screwed everything up for my September-born kid! Doomed from birth, poor thing!

Oh boy another thing to strengthen my imposter syndrome

Read the book “Outliers”. talks about this exact thing. are you really naturally good at soccer? or were you born at the right time and thus were a step ahead from the get-go

Which month is the "right" month definitely changes by culture, but the premise that being the oldest in your childhood cohort confers an early advantage seems to hold across multiple countries.

If it makes you feel any better, the magnitude of the advantage conferred isn't huge. I'm a September baby, was almost always the youngest in my class, and (at the risk of bragging) I'm objectively successful. I was never great at sports, but being several inches below average is probably the culprit. :)

What about countries in the southern hemisphere, where the school year starts in march?

Is birth year more culturally significant in schools there? Here it's not really even a consideration. Ofc the school starting cutoff is usually based on age at the start of the school year in august iirc, not on calendar year, so I had a mix of classmates born in 94 and 95 for instance. Being born in summer made me one of the youngest, and being born in december would place someone in the older half. We only really associated with grade level, and with graduation year (both high school and college). And assuming your sports are through the school system (most are, but not all, and those that aren't are usually more chill) I don't think actual age affects eligibility at all until it's like state and national championship level U16, U18, U23, etc categories.

Also, an important distinction in this is that those 11 extra months of growth isn't just physical size. It's 11 extra months of using their body and mind in everyday life, gaining coordination, balance, depth perception and all the other physical and mental components that go into being a good athlete.

Player A has had over 10% more time alive for their brain to develop than player B by the time they start playing their sport.

Malcolm Gladwell bruh. Outliers

The month doesn’t matter lol. It’s the phase the moon is in obviously.

All the best players were born on a blood moon when Saturn and Uranus were lined up(common knowledge).

This is looking at a single variable accounting for nothing. Basically a small linear correlation in a small sample size. Wouldn’t look into it too much

Read that from the book the Outliers

Same birthday as Neymar Jr and CR7, but I can't even dribble the ball.

I was the youngest in my grade, and I skipped a year. I think it was an advantage to start college at 16 (many start even younger)

I don't really think it'd be correct to say it's 100% a question of maturity and nothing else. Things like genes, nutrition and health are examples of factors that could be playing a part. Loads of things contribute to a child even getting the chance to start playing a sport, which is a requirement for them to be good at it. To reduce all that down to differences in "maturity" is a bit simplistic in my mind.

Values ​​that span multiple order quantities. But instead we have a fixed set of values ​​(Jan-Dec) with no positional notation. If we try to convert and apply the months to their numerical values

Of course you are correct. I'm just saying that the MAIN factor in determining if someone will be good at sports is literally how physically mature/how big they are. (As a kid, "size" is pretty much determined by physical maturity (and to a lesser extent, the things you mentioned).

Some dirt poor, poorly fed kid who is 5'6" (due to maturing quilckly) is still going to destroy a rich, well fed kid who is 4'10" tall in any physical sport.

Genetics and what not will determine the final height of most people, but the speed at which one matures will determine how quickly they get there (and therefore, how much larger they are than their peers when they are young.)

And let's be honest here, MOST children who will be playing sports (in the US) are fed well enough not to affect development.

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Yes! I have read it.

That's because in sub 20 or sub 18 etc, when you are more grown you have more advantage, so it's more usual to have the bests (usually more grown, like January February and 1st months of the year) the continue playing till being professional

It is like that in schools, more grown folks develop better in schools, and if the groups are made based on birth year, people who are born in 1st months are less likely to fail the course than their last months counterpart

Ye, snowball effect

This is nothing new if you've read Malcolm Gladwell's book Outliers.

He just covered this again in the Revisionist History podcast. The age effect is definitely a thing.

I have, but as you know he did not include data on the 2022 World Cup

Some recent studies have shown that holding boys back a year is actually beneficial to their success in school, while starting girls at the usual age is suggested, because girls mature faster than boys. Several friends and relatives who have recently had children are seriously considering holding back their boys and letting the girls go as prescribed by the school district. Based on my own children, this seems entirely on target.