Prime isn't an age. Its when your skills are at or near their peak. Anderson Silva is smack dab in the middle of his prime until someone beats him.
Frank Mir comes to mind. Technically Anderson is past his prime and is still VERY relevant! I suppose we would have to define "prime"...
Nog
Fedor
Cro Cop
Couture
Serra
Hendo
Shogun
And before you say anything, prime does not equal still winning fights. Prime for me is when skill, speed, strength, and athleticism are all still rising and none are declining.
For example, Nog was in his prime in 2002. Cro Cop was in his prime around 2004.