I think that there isn't a general rule here and it depends individually from fighter to fighter, but I also believe that there are 3 most important causes that put most fighters "past their prime":
1) Age would be the most common and obvious. Father time gets to all of us inevitably and, no mater how gifted or competent an athlete may be, no mater what kind of training he does, sooner or later it's all bound to diminish. Skill can be maintained through training on the highest level, but speed, reflexes, reaction and other athletic attributes can't and, in time, they decline with everyone.
Best example of fighter past his best because of age at the moment would be Vitali Klitschko.
I mean, yes the man is still winning, but look at what he looks like in the ring. His prime was 2003/2004 until the first retirement. I don't know how well you remember him from back then, but that man was a killer. How he handled him self in the Lewis fight and then those hatchet jobs he did on Kirk Johnson (one of the most electrifying knockouts I've ever seen), Corrie Sanders and Danny Williams were really something impressive. He was fighting with real aggression and coming to take people out then. And now you have this old, slow paced, ultra measured, methodical fighter with almost no explosiveness or dynamics of any kind. Watching him work hard and having to struggle with guys like Dereck Chisora and Manuel Charr - it's just clear to all who have followed him that he should call it a career.
2) Wear & tear and injuries are also very important factor. Some boxers aren't old but they can get through some vicious wars in the ring, which take toll on their bodies and cause irreparable damage. This shortens careers and, unfortunately, in many cases reduces the quality of their post boxing life.
Of current fighters Michael Katsidis would probably be the best example. Late Arturo Gatti is another and there are of course many more.
3) One fight beating is the third cause. This could go under the second group, but sometimes, as you know, all it takes is one match and punishment sustained can be enough to crush a fighter and ruin his career.
For example, Pipino Cuevas was only 22 years old when he fought Thomas Hearns for the welterweight title. All it took was 2 rounds and he was never the same again. Hearns literally destroyed him as a fighter in under 6 minutes. Joe Calzaghe vs Jeff Lacy is another example. Lacy's career as an elite boxer, at the age of 28, basically ended in that fight.
When the boxer loses the desire to give 100%, that's the start of the 'decline'
The age factor can be dealt with, you only have to look at Hopkins & Klitschko to see that.. As long as these guys keep giving 100%, they can still mix it with the elite.
Once the motivation had gone, the will is weakened, a mans resolve can be tested and broken.
Skills are maintained and enhanced in training, once the dedication has diminished, the cracks start to appear, once they appear, you either quit boxing, or succumb to a beating.
Example;
Floyd Mayweather - Trains 24/7 Totally dedicated to his craft - Winner
Manny Pac - Brilliant boxer / Full time politician and congressman - Regularly burns the candle at both ends... Gets KO'd against opposition he has beaten several times previously.
Age would be you're getting to old to do something and skill wise would be if you were good, really good, and then on a slow decline
well with age you lose your reflexes and also depending on how much punishment youve taken in your career depends of a fighter style of fighting the less you get hit the longuer you last
Give examples of each...
