In boxing it's hard to consider a fighter's prime. Usually it means when a fighter is at his best. That's the general consensus, regardless if he's in his physical prime or not. Meldrick Taylor was one of the best talents in boxing I've seen. But I remember watching the HBO documentary on his fight with JCC and they said something like this, "A fighter can lose years of his prime in a brutal fight like that." And Taylor was basically a shot fighter at around 27 or 28 because of too many brutal wars. Even though Taylor was in his physical prime he was past his boxing prime.

ANother example would be Muhammad ALi who was in his physical prime at 29 against Frazier in their first fight but was actually somewhat past his boxing prime because of the long layoff and he was never the same Ali of the 66-67 one before he was ban.

Another example would be Bernard Hopkins who I felt was in his boxing prime when he was 30 years old and that's when you start losing your physical prime. Hopkins was just a better fighter in his 30's than when he was in his physical prime of his 20s.

So imo "prime" means when a boxer is at his best, and it doesn't have to mean in his physical prime.