It's the same thing. They don't peak at 21 because they still have improvement in them. If they leave the game by 21 it's not because they peaked at 21. It's because their junior contract ran out and no club offered them another, or because they didn't want to stick around earning $50k as a backup. But these days even the lowest paid top 30 players earn a living wage so they can stay in the game and continue to develop.
I said "any player who remains a full time pro", which you even quoted. Now you're trying to misrepresent me, on a forum where our words are on record you silly merkin. Don't be calling anyone else silly until you sort your own shit out.