He is 41.
For reference, when I was 18 I ran 2 miles in 10:32.
(5:16/mi pace)
Age 30-35 I struggled to run 9 min miles
45 now and I was only running 10:30 miles after training for 2+ months. (I'm slow. I know)
So literally I was twice as fast in my athletic prime as I am in my 40s.
I am not fat. It is just age.
The body slows down a LOT from 35-40+
Strength and power actually holds up pretty well.
Speed and endurance plummet.
Especially if you have injuries and surgeries, which would be extremely likely for an athlete.
His buddy/brother in law (I think) Chris Weidman is at 20+ surgeries at roughly the same age.