Prime isn't really a gray term.... Cotto isn't prime. Most would say Pacquiao's prime was 2007-2009. (the peak of his abilities & physicality) I'd say Floyd just before he moved to 140. Some fighters do blur the meaning of the word as their physical decline is outweighed by their increase of skill/mastery of their abilities.
which is your opinion, not the answer to an equation. making it a grey term. you can be more athletic at 25 but have a better adjusted defense at 35.
It's easier when we're not dealing with such great fighters. It's less hard to identify the prime years of grade B or less fighters. Would that be agreeable? The rise and fall is usually apparent. Good luck working out who i'd classify as a grade B or less fighter though. Damn this shiz is getting complicated. Back somewhat to your original point - Yes, It's all totally subjective.
Seems like itsa has an agenda. If we are just gonna talk the term prime, then yes I agree it is a gray area. However prime used to be easier define in previous boxing generations because almost everyone would start young, fight 100 fights or more, and be done by age 25. Mainly the boxing business and culture was different back then. Now we have pay-per-view champs who are making bank, taking care of their bodies, hiring nutritionists, strength and conditioning coaches, fighting 2x a year. It's totally changed the game and made it slightly harder to tell which elite fighters are done and who is still competitive. However, the business side of things mask competitiveness as well. Too much ducking, too much hype for no hope fights. Pac should have never fought Rios or Algieri. Cotto's has kind of come back, but were Rodriguez and Geale really ever serious threats? When it comes to Mayweather, he's done a lot of ducking and dodging in his past which prolonged his career, but I wouldn't consider him prime now. If someone beats Mayweather now it really wouldn't mean the same thing as it did 5 years ago.