The talent pool being small is part of it, but there are other factors involved. The type of athleticism involved in MMA is very different from the athleticism that is required in sports like basketball, football, soccer, or even wrestling to a certain extent. Men continue to get stronger, well into their early thirties, ever heard of old man strength? Just look at sports like weightlifting and strongman. Brian Shaw is 37, Bjornsson is 31 and getting stronger every day, Lasha Talakhadze who is turning 27 this year and is slowly becoming one of the greatest Olympic weight lifters of all time. Look at the 2016 Olympic Wrestling field. Some young guns aside, there are also plenty of wrestlers competing at the highest level in their early thirties, and some even older in the heavier weight classes. People often conflate athleticism with how fast someone can run, how much weight they can lift, or how high they can jump. These are literally all secondary aspects of fighting.
The reason you see so many older athletes in MMA is because the sheer amount of time it takes to learn all the necessary skills. Many of the top level UFC fighters spend their childhood mastering a martial art, whether that's wrestling or some form of fighting, and this martial art acts as their base for transitioning into MMA. It takes several years to learn just one skill such as Boxing, Muay Thai, Wrestling, or Jiu Jitsu, much less several. If someone starts transitioning to MMA from their first sport at the age of 25, it's only natural that they'd really start to hit their stride right around 29-30, and having that last for several years.
My point of saying all this is to address all the people claiming that Jon Jones is no longer in his prime just because he's hit the age of thirty. The reason Jon has been struggling is because he's being closely monitored by USADA and is no longer able to juice himself up to the tits, along with the fact that he's finally fighting taller and longer opponents, which have always been the kind of fighters that Jon has struggled with. Jones is one of the most overrated fighters of all time. He's great, but certainly not on the level of prime GSP, Fedor, Aldo, Khabib, etc.