That's a debatable topic that I've heard many different sides of. Most larger athletes fall into what you're suggesting... but most endruance based athletes peak in their late 20s/early 30s and MMA is without question an endurance sport.
As far as the general concensus around here goes. I actually think Carlos as the best chance of beating GSP, and should Hendrix beat George, and Carlos wins his next fight or two... that is a very promotable rematch, the Hendrix/Condit fight easily could have been scored the other way.
It's funny that Diaz was bitching about the scoring system after his fight (as he always does I know)... but the truth is Carlos had the most reason to be pissed, but he's too classy to bitch.
On average, test levels start to drop after 25.
Take a look at this page with stats from the 2012 olympics.
Over two thirds of the medalists were between the ages of 20-30. And I would be willing to bet if you cut out the non-athletic events like archery and equestrian riding that percentage would grow.
ETA specifically, take a look at the average age for wrestling (26), judo (26), boxing (25), and taekwondo (23).