I found that the problems I was having often had to do with the decrease in flexibility that came along with age. That decrease in flexibility also lead to a loss in power. I found that increasing flexibility training (like that comment about Yoga) helps to decrease the rate of deterioration. I have also taken to the use of nutritional supplements and other drugs to aid in recovery from heavy workouts. I have to daily take my 500 mg of Glucosamine , 400 mg of MSM, and 400 mg of Chondroitin 3x a day. Otherwise, I am looking at knee replacement. However, I have found that the drugs are not enough and have had to increase my leg and hip joint flexibility exercises. Another key area that I find I must now work on continuously is back flexibility. Ukemi are harder without that back flexibility. And lastly, is shoulder flexibility. To be able to put power into your techniques requires good shoulder flexibility. (So does my golf swing.) So that is another area that I have to work out. And the last is wrist flexibility required for Kaeshiwaza. The warm-up that I lead in classes now are as much for me as for the students. Oh, yes, back to hip flexibility required power (and swinging the golf club). I find that most people do not do the fune kogi undo properly so as to improve hip flexibility.
As I get older, I am finding out how important all that flexibility really is. You don't miss it until you lose it.
I think that if you want to keep practicing hard and enjoy your practices, I suggest working on your flexibility. Even with damaged bodies like mine, you can regain some of the flexibility of your youth with hard and continuous practice. By the way, my knee problems have decreased substantially with increased flexibility of various parts of my legs and hips. It seems that the rest of the body can help accommodate the loss in knee power and flexibility as long as the rest are more flexible.
I have found that as I age, I have had to increase the number of my practices to maintain my abilities. This increased number of practices is not at the same speed as when I was younger but tend to be more intense for a shorter duration. In other words, I haven't cut down the amount of pounding on my body but I have spread it out so that I don't have to recover so much. I notice this from golfing tournaments. Playing four games in a week is not difficult when spread out over the week but playing four games in three days takes a real toll on my body, especially when partying every night during the tournament.
Another thing that helped -- moving to a warm climate. The Canadian winters were killing me and my joints and the driness was murder on my skin. Barbados is wonderful! (As I sit here in the 29 degree warmth.) It's much better for your golf game as well.
BTW, most people in my Caribbean dojos are over 40 and many are near 50.