No it isn't.
Top MMA fighters are in their 30's because the talent pool in MMA is incredibly small so older fighters can still be successful. There are not hundreds of elite level 20 years olds coming into the sport every year like in other bigger sports.
there is also survivor bias. The top guys are still good at in their 30's because they are the top guys. They were already better than everyone else so they will take longer to decline. They are the exception, not the rule.
You are ignoring all the other guys who didn't make it into there 30s. They were out of the sport long before that.
It's like saying 35 year olds are still above avg NBA players because LeBron is. Or that all 19 year olds can play in MLB because Bryce Harper could.
https://en.m.wikipedia.org/wiki/Survivorship_bias