Shane Carwin will fight at UFC 96 against Gabriel Gonzaga. MMAFanhouse has an interview with the rising heavyweight. Here's a quick quote about Antonio Rodrigo Nogueira,

Watching Antonio Rodrigo Nogueira recently, he's only 32, but he looks like an old man, like all his fights are catching up to him.
Yeah, when you're sparring three times a week, and taking those kinds of shots competitively, everyone who fights, there's always something that's bothering you or nagging you. That's just the nature of the sport. I think that's why Randy Couture's been able to do it as long as he is, because he got a late start. I was just talking to one of my trainers about it. People really only have 10 or 15 years in this sport before they start showing the signs of age.

Carwin raises a good point about the decade time frame. There is a lot of statistical evidence that professional athletes have, on average, at max shelf life of ten years. The best years for an athlete are usually 24-28. Now it is a little different for a combat sport, due to the need to climb the ladder within a division.