Bolt lost to Gatlin, who's five years older than him. Jordan was outplayed at the end by several older players, as was Gretzky.
Or look at Roy Jones Jr. When he and Hopkins were in their late 20's, Jones won easily. When they fought again in their forties, Hopkins won easily. Now most people would say that their fight in their 20's was in their primes, so Jones's prime was better - and in fact just about every boxing fan will tell you RJJ was one of the best ever, better than Hopkins. However, on Sherdog people will say 40's Hopkins exposed 40's RJJ, and so it proves that RJJ was always overrated. Just as Gatlin, beating Bolt when both where in their 30's (and Gatlin is five years older), exposed Bolt as being overrated.
The point being, athletes age differently, for a variety of reasons. Look at Shaq in his mid-30's, being outplayed by quite few guys his age. Some guys peak earlier. Anderson lost fights in his 20's, when Fedor was undefeated. He won fights in his mid-30's when Fedor started losing. Both had a good career of ten years, but their top performances started at different ages. And in fact a lot of analysts will say length of career is the magic number. You might be at your top from 22 to 32, or 27 to 37, but its always going to be about ten years. Only folks who do heavy PED's tend to beat that in contact sports. In the case of MMA, most of the wear and tear comes not from the fight themselves, but from training camp (and yes, that's a real problem with the way fighters train - in no other sport do you have so many top athletes hurt in training; any NFL coach who had repeated stars hurt in practice would be looking for a new job).
In any case, there are very few athletes whose performances don't drop off at the end of their careers. Almost every top athlete in every sport does this. With the exception of a few guys who quit while they're young, everyone is exposed. My guess is that the reason MMA fans don't get this is because its a new sport, and for many its the only sport they've ever watched, so they're not used to seeing superstars fad, and hence they think fading with age or injuries means being exposed.