I am conflicted by the NFL/Heavyweight Boxing theory. Both sides make great points.

[CubanGuyNYC]

lol I rewatched Sergio vs Dzinziruk the other day and they mentioned something like that and look how that turned out.
Sergio starts at 20 and ****s up the lifelong boxer

Guys like Sergio are the exception, not the rule. But your point bolsters my theory. As "dc3383" points out, you can't take up football or basketball in your twenties and become successful. There's very little space on professional rosters. Only the very best get signed. Even the worst scrub on a pro team is a relatively amazing athlete. There are a lot of terrific athletes that don't make the cut...tons. We'd be seeing a lot more of them in boxing if sheer speed, athleticism and strength were guarantees of success in the ring.