I'm not saying players that are good at 12 won't be good in hs and beyond.
But there are players who good at 12 that have had more training than others. Does not mean they will continue to develop
There are players who bigger and stronger than others who may not continue to develop as players.
Just has there are small players who will grow, and players will who just started playing at 12 that will be come great ball players.
None of the All Stars on my son 12yr LL team played beyond hs, most did not play in hs except my son and he was not considered a stud as a 4'9" 12 yr old