Meldrick Taylor: prime years 21-23, past his best at 23, a shot fighter at 25-26 Wilfred Benitez: prime years in his early 20s, a shot fighter by 25 Mike Tyson: peaked at 21, prime years from 20-24 Bernard Hopkins- prime years in his early and mid 30s, still very effective at 46 Glen Johnson: when exactly was this guy's prime? He is a physical marvel too.
In Benitez's case, the fact that he turned pro at the very young age of 15 would have a lot to do with his, he had a good ten years as a top fighter, which I would consider a normal amount of time for a fighter.
Glen is one tough mofo, I doubt he'll ever be out of his prime I believe the style he fights makes him so good and doubt any age more will change this.
Bernard Hopkins- Old (35) Wladimir Klitschko- Old (33) Archie Moore- Old (35) Mike Tyson- Young (21) Floyd Patterson- Young (23) Muhammed Ali- (relative) Young (24)