I know....
I'm 36 so sad to think that I've missed out of the prime athletic years of my life. But truthfully I am probably in the best shape of my life thanks to jiu jitsu. I can definately do things that I could have never done in my 20's because I didn't work out back then and took my youth for granted.
However, at the other end of the spectrum, there is my professor, who is probably in his 50's but holy hell can that guy smash the living shit out of anyone. I've been there 2 years and he has never been tapped, Ever. Even against other visiting black belts who are much younger than him. Just a hard, old school, grizzled kind of guy. He's not big at all either, my height and probably close to 185 pounds or so. But he smashes everyone, including the 230+ pound behemoths we have.
So there are exceptions.... I dont think the guy is genetically gifted or anything, just after 30+ years of BJJ his body is just really fine tuned to smash people. This stuff does harden you if you stick to it long enough.