First, heavyweights are older just because it takes longer for these guys to "grow into their bodies" if you will. You just don't see as many young star heavyweights as you do in other weight classes. Tyson was the exception, not the rule. Same with Ali. Sometimes it takes several years for heavyweights to put everything together.

As for the lack of American heavyweights, the answer is simple. If you're a great athlete and you're over 200 pounds, why would you want to be a boxer and be forced to deal with crooked promoters and get punched in the head, when you can join a unionized sport like the NFL?

Everybody's looking for the next Ali, Foreman, Frazier, Norton and they're all out there. They're just named Ray Lewis, Michael Vick, Terrell Owens, LaDanian Tomlinson, etc. 50 years ago, those guys would have been fighters.
And Allen Iverson would be challenging Floyd for the welterweight crown. Times change.