I think it has more to do with girls not developing their legs, hips and thighs until they are a bit older. A lot of girls in high school had big racks, but rest wasn't really developed.
I always wondered if I was going to grow up to be one of those creepy guys that finds really young girls attractive, but thankfully, i'm normal. I'm still in college now, and not much older than the kids in high school, but I went to pick up my brother from school and the girls looked like little kids. I could tell which girls were the attractive ones, kinda like how us straight guys can tell which guys are attractive, but i had no urge to want to fuck them.