Depends, Tyson, Armstrong, Ketchal, Battling Nelson, Terry McGoven all had short primes. And most were shot by the time they reach there late 20's early 30's. Than we have guys like Joe Walcott, Hopkins, George Foreman, Larry Holmes, that seem they can go on forever.
boxing is different from other sports in that a 29 year old can be fresher then a 22 year old on the "prime age" it depends on a lot of factors mainly, the style of a fighter (a slugger is ganna age quicker) the amount of fights, level of opposition and skill set its also worth noting that fighters develope at different stages physically and mantally
depends on the weight. a light weight fighter at bantamweight is old at 26 and a heavyweight if he has power is not old at 40.
it depends. there are some boxers whose physical prime might have diminished but is at the peak of when it comes to their boxing ability. look at juan manuel marquez and bhop