What is the definition of "the prime of a boxer"? When is a boxer in his prime?

I guess it all depends on the boxer some peak before others

I think late 20's/early 30's are generally the prime for a boxer provided they haven't taken too much punishment early on. It's when they are still at their physical peak and they also have the essential experience and IQ that they lacked at a younger age, it all comes together. Of course there are a huge amount of exceptions, this is just a generalisation.

Kinda depends. Like Bernard Hopkins; I think he was at his absolute peak/prime at 36 years of age. It was when everything in his game came together (physically, mentally, ring IQ, etc). I'm guessing he may have been physically stronger & a bit faster when he was 25 years old, but he wasn't nearly the fighter at that age and thus not considered prime at 25. See, I don't believe that to be the case with Hopkins in terms of ring IQ, I didn't see anything with Hopkins in his mid 30s that I didn't see earlier in his career. Just that Hopkins, fortunately for him, was always physically strong that he can always manhandle his opponents/ hold his own and stop them from getting off when he no longer had the stamina. All that changed was that he became more of a spoiler, not that he didn't know how to apply any of that before. Hopkins physical prime on the other hand is harder to spot, I can only say that he started to slow noticeably after Joppy. It's weird because with Hopkins how much of it was getting down to 160/ age?

When they're at their absolute best physically, the ending point is the hardest to spot. I do consider chin as well, those who tend to get hit a lot lose their prime early. I personally don't care for whether they were still hungry or not, that should not count.