I started practicing martial arts at the age of 21. I could go all night back then. However, although the spirit is still willing, the body has succumbed to the effects of age. No matter how hard you fight it friends, everyone ages; and aging affects what we are able to do.
I am very thankful for what I can still do. Actually, the truth is, as far as techniques go I can still do pretty much everything I ever could. However, some things are harder than they used to be. High kicks in particular seem to take more effort to accomplish than they used to. I am probably as flexible, or loose, as I ever was. But it is harder to do some stretches because I have a belly that I didn't used to have. And I can't seem to get it off of me. Sound familiar?
It could just be in my head, but I also think I'm a little slower than I used to be. Don't get me wrong, I am still pretty fast. But I used to be lightning fast. My speed was one of my best weapons. I get hit more often than I used to when charging now so I'm convinced that I must have slowed down some.
My biggest complaint though is the decrease in my endurance. I've said before to friends: I'm as good as I ever was, once. My endurance has diminished to the point that I can no longer keep up with the regular class pace. I can do the same workout, but I have to do it alone and at my own pace which is slower. I have to take more rests more often to accomplish the same thing I always have. This is kind of scary to me. In today's world it is more dangerous than it has ever been. And there is no respect for the older generation from the youth. Not too long ago I had a young man at least half my age and twice my size bow up at me for no reason that I knew of. Thankfully I was able to walk away. But if I had not been able to walk away, and would have had to defend myself I would have had to do it quick; real quick, because I don't have the stamina for a drawn out fight anymore. I'm good for about two or three minutes of hard fighting, then I'm spent. It don't matter how good you are, when you get tired you're beat. That is true at any age.
Anyway, that is the major things that I have noticed as I have gotten older. My advice to you is simply this: Slow down your workout but never quit. Take how ever many rests you need. Stretch slower but stretch. Stay in the game because even if you are slower than you used to be, and it's harder to do some of the same things, you are still benefiting from your workout. At some point in time it quits being about self defense as much as the health benefits anyway. Train hard, train regular, train smart.