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).
While those figures are probably correct, I don't think MMA works quite like any of those individual martial art sports.
When you start mixing these sports together you move beyond simple speed and strength issues where youth plays a bigger part, and into a sport where experience and strategy plays a more important part, which comes with age.
While it may be that a young boxing/taekwondo practitioner might have an advantage over an older boxing/taekwondo practitioner, when it comes to becoming proficient in many martial arts, and knowing when to (and not to) use which technique, then the older person may actually have the bigger advantage (up to a point anyway. Maybe moving that 'peak' a bit past the mid twenties to give MMA fighters in the thirties a fighting chance.)
That's my opinion anyway, but it may be a bit outdated in that GSP may have 'moved the bar' a bit so that now the younger fighters are learning earlier that training in all the MMA disciplines, having a good strategy, and controlling your emotions is the key to winning, and not just being great at one aspect of MMA .. so now the younger fighters, with they better training, and the advatage of youth may make the days of 35-40 year old champions a thing of the past.