depends on the fighter but generally, on average for all divisions, 27 has always been the number that I heard. Lighter weights generally prime sooner and heavyweights later. Todays fighters have turned convention on it's head. Fighters the age of Pac, Mayweather, Ward etc.., etc.., all are at or way past the age where even great fighters had to to struggle or took ass whuppings from lesser fighters. It's true that knowledge and experience continue but the physical decline has generally been considered to be well underway by 30. So, the experience eventually cannot outweigh the loss in reflexes, in endurance, in durability in speed. I've heard so many fighters, Ali and Alexis Arguello were eloquent about how they said it, that their experience made them better, but it wasn't true. Even when you know what to do it doesn't do any good if you can't execute.
One more thing, I think "Prime" means different things, often times, maybe most times, fighters peaks do not correlate to when they get their defining battles and also they aren't always at their peak popularity. Ali was a very unpopular champion in the sixties but he never had greater abilities, he also didn't have the defining opponents so, physically, he was never better than he was in the 60's. In the seventies, he lost his legs but he still beat better fighters and he was more popular. so, in terms of popularity and accomplishment, you'd have to say 75 was his prime in those areas. Hagler was popular after Hearns but he was slowing down drastically by then, he was losing his hand speed way before those years but he was still improving in so many other areas. Lots of examples of what I'm talking about.
