I can't imagine what kind of HS experience a kid would have, senior girls would not date a freshman, freshman dads won't let their daughters date an 18 year old. By senior year a kid held back that much will be able to by booze, etc., and he will be more emotionally developed than his classmates.

Does this happen in the sports where there is an actual pro-sport outcome with multi-million contracts after college?

I know hockey players wait a few years after HS, play junior league then go to college, that system makes more sense academically and socially, finish HS in the correct year and take some classes at the CC so you can have a lighter course load when you start college.