Jim Lampley said before the start of the Martinez fight announcing the age of Martinez and Dzinziruk at 35 & 36, that the mid thirty's is now the "New Prime". He cited his evidence was that fighter's conditioning was getting better and better at this later age.
Personally I think this was is just hype BS.

I think there are two aspects to a fighters peak. 1. the maturity of his boxing skill. 2. his physical conditioning.

I think a fighter is at his optimal peak is when these to aspects are at their best and I also think many fighters never get to that point or one peaks before the other. George Foreman hit his peak physical conditioning as a young man, but was able to mature his boxing ability to fit his old body at a later age.