People have different definitions of "prime". I define a fighters prime as the peak of their career where they are able to demonstrate physical, mental and technical (skill) at their very best.
Physically your athletic prime is generally considered to be in your early 20s to early 30s (depending on the sport) and slowly decreasing with age, along with your test levels, reflexes, balance, flexibility etc. For example, majority of female gymnasts peak very early on. You generally don't see female gymnasts compete successfully in their mid 20s because of injuries and the loss of flexibility and balance. Of course there are very rare exceptions, but it's extremely rare.
Technically your skills tend to improve with age. you gain skills through practice and experience in the cage, granted nothing mentally damaging happens to you. I would assume you could continue to improve throughout your career, tho, some people will peak technically early on and deteriorate over time or reach a learning hump they can't get over.
The mental factor is something real hard to gauge as spectators. Some fighters can get KO'd one time and will never ever recover from it. It's something that will always be in the back of their mind regardless if they are successful or not. Other fighters do not seem to be affected very much by a KO. During the Bisping fight, Anderson Silva said he thought he broke his leg again. For the rest of his career Anderson Silva will probably be afraid to throw any hard kicks. Donovan Ruddock believed he was never the same fighter again after two grueling fights with Mike Tyson. And some fighters just aren't there 100% for whatever reason.
Then there are other things like time out of the cage, bad game plans, and injuries.
Anyway, prime can be considered many things and it's something that is real hard to quantify due to the number of factors involved. I think JDS looked pretty bad coming off his first loss to Cain and some of his sequential matches, but it's honestly hard to say what the hell is going. Could he just of had a bad game plan? Did he mentally break down and was slowly trying to get his confidence back? Did he fight a superior opponent? Maybe. But all I know is he is getting older and he has been injured and we can only gauge their current situation by how they performance from one match to another.
