Because toddlers and babies don't use something like TikTok and there is no upper limit on the age to skew the average upwards. There is also the problem where a lot of you people stick in fake ages. Back on point, though. You don't need a 10k person to skew the age up. I gave an example in another comment where a couple of 46 year old people can skew an average with a majority school aged group up to 22. Here it is:

If you have 5 x 13 year olds, a 15 year old, a 16 year old, a 22 year old and 2 x 46 year old people. That averages out to 21 years with the median being 14 years.