Most of those kids reach thier peak at an early age, then don't pan out at the next level. I played against a kid in junior high that was a beast at running back (speed, size, power, he had it all). He ran all over our team in the 8th grade, for like 230 yards and 3 tds. BUT:
In high school, he didn't do anything like that. He was good in track, and played safety on defense, but was only a second string running back for his team. A couple of other track kids were better at learning the more complicated high school playbooks.
We got our revenge for the 8th grade butt-kicking, we beat those guys four straight years in high school and we were state champions our senior year. We shut them out 42-0 the last time we played them. I can't help think that that kid should have made more of an impact in high school.