At 41, I just take it easy and enjoy myself. Nothing to prove. I like to say that my goal isn't to be better than you. It's to be better then I was yesterday.
What we all teach, and what we all learn, is - "do your best." The physical qualities of your "best" changes as the body ages. You still play the cards you have the best that you possibly can. You just don't have aces anymore.
I'm 45 and will be 46 in a couple of months. I've been in Kenpo on and off since 1985 and still a brown belt. I've recently had surgery last month to fix a rotator cuff injury and currently rehabbing with physical therapy.
It is tough watching my body age through the years. The hardest part now is that many people are pressuring me to retire from Kenpo now--especially since I've had a serious injury. I really don't want to, not when I'm this close to being a black belt and that there is more after becoming a black belt. I think I'm a better martial artist now than I was before--it is much more than just the color of a belt.