I've noticed as I've gotten older that the body needs a bit more time to bounce back between workouts. When I tested for first dan (around 34 or so) I was quite proud that I could keep up with the youngsters. 13 years later and I have had to accept that while I can still do most things, it just takes a smidge longer to do them. It makes me realize that martial arts may be the best way to stay in tune with your body. The body is the best indicator of all things from physical health to emotional well being. I pledge never to stop my martial arts training. Weight lifting may be beneficial but the physical training of martial arts just keeps you in balance and in tune much better.