Three of the very best 12yo baseball players I've ever seen fizzled out in high school. They were faster and stronger than their peers and were amazing at 12, I thought they were without a doubt on the fast track to the majors. Now when I hear stories of phenoms I wonder...
Jordan was no phenom at 15, Lebron was and Pippin was not. I would guess that a larger percentage were studs at 12 in their sport, 6-7 of the ones playing in the minors that I know were very good at twelve, they just weren't the studs that caught everyones eye.