Everybody ages differently, but I think Torres is one of the best examples of a guy losing it before he should have.
Another one is Kid Yamamoto. 17-1 in MMA wins over guys like Sudo, Uno, Royler, Yahya, Bibiano, Curran. Great standup, some of the best ground n pound ever in the sport and a phenomenal wrestler. Ends up hurting himself (shoulder?) trying out for the Olympic team at 31 and was never the same since. 1-5 since then and hasn't even shown a glimpse of his old self.
If we had TJ, Barao, healthy Cruz, Faber and prime Yamamoto right now at 135, it would be the best top 5 the sport had ever seen.