I think with any fighter thier prime all depends on who you talk to. A fan, media, the fighter himself etc. Basically the way it's used most often is if a fighter is winning he's in his prime, when he looses to someone better everyone says he's past it. It's usually used for people who like certain fighters as well. If a guy wins who you don't like, just say he won cause his opponet was past his prime. This term is used more as an excuse than it is a measure of the condition of a fighter.