The DSM IV had the rate at "1:10,000 to 1:100,000 for MF and 1:30,000 to 1:400,000 for FM". That's from back in the 90s to the 00s. The 1 in 50 comes from gallup. If you look at the gallup data, scroll down to where they break it out by age, as it's .2% for GenX, while it's 1.8% for GenZ.

Of course, GenZ isn't teenagers. One study from pittsburgh showed a rate of 9% of kids reporting as either transgender or non-binary, so getting close to 1 in 10.