Being a gymnast requires an insane training schedule and an extreme level of flexibility and fitness; part of this means maintaining a very low body fat percentage. When young females drop below a certain body fat percentage (around 15-20%, if I recall correctly), hormones go crazy. They stop growing, their periods stop, amongst other things. That's why those 20 year olds look like they're 13, and the 16yo looks like she's 8.