A fighters 'Fighting Prime' does not generally correlate always with their 'Physical Prime'.
If it does then Anderson was losing a lot of fights to lower end fighters in his Prime as man's physical prime is around 25 years old. Anderson was losing to Takase at 28 and Chonan at 29 years old.
So you cannot have it both way Android fans.
A fighters 'Fighting Prime' is when his mental game and physical game meet at a point of their best execution.
I would peg the up slope of Anderson's physical prime in around 2008 after that sloppy fight with Lutter where if Lutter did not have such a horrid weight cut and gas out he likely would have finished Anderson. I think Anderson's complete game came together much better after that. And I think Weidman knocked Anderson out of the down slope of his 6 year prime run (which is not a long time) with that devastating KO, which happens.
A prime is not only about age, physical state, or how many fights u had (all sports combined)..
It can be a mental thing, like Frank said after the Cung Le fight, when he said that he no longer had the mental discipline to follow a gameplan.
A prime is also not about winning or losing. U can still be winning and no longer be in your prime: this is called fight IQ & experience, mental warfare too (see the B-Hop case).
Now a physical prime, back in the days (now with so many sophisticated PEDs...), would go up to 34 yrs old maxi.
What´s happening in some UFC divisions is obviously an anomaly.
The way I see it, Spider in Pride was quite an anomaly: fighting @ WW, obviously in his physical prime, some would argue not in his technical prime (which is a reach, since he already had arguably the best ground game at Chuteboxe according to the Pride commentators [the truth is...Assuerio Silva was perhaps a lil bit tighter]).
The Chonan & Takase losses are legit, no fluke, even in his technical prime he could have lost these 2 fights since the 2 techniques were beautifully executed by 2 underrated fighters.