Bruce,
While we are waiting for more informed people to reply, I'll give you a "I feel your pain" kinda thing. I'm 59, in fair shape for my age and have been an exercise nut for about the last 20 yrs.
I think we all reach an age when our bodies require more rest to recuperate. Genetics may allow some people a bit more leeway, but I am now firmly in your camp... I've had tendonitis three times in the right elbow in the last 4 yrs., my back will occasionally flair up and I've already had one back operation in 1998 (which ended my squating), and I have a small tear in my right rotator cuff..... all this (and compared to many others I'm very healthy) is manageable with common sense. I've had the discussion with instructors and doctors...you need to balance your activity with what your body will allow. If you have tendonitis... you've done too much of something and you need to rest a bit, but more importantly you need to recognize what led to it and avoid doing that much. I try not to work out very heavily more than once a week... I still work out 4 times a week but the other three are more moderate... I hit balls or play 5 days a week...I'm tempted on the other two days... but I'll just do mirror work, or hit the impact bag or work with the dowels. If I don't get a good's night rest for some reason, I take it a bit easier the following day.
I don't recognize the added rest and recuperation as a concession to my age so much as just a necessary evil that allows me to do as much as I possibly can. Every workout (or activity), even brief abbreviated workouts, are good...injuries prevent you from being active... you have to avoid injuries especially as you reach your 50's.... just my opinion.
While it is entirely possible you are right that tendonitis is a sign you need to slow things down, Pete Egoscue, author of Pain Free believes most pain syndromes are a result of biomechanical dysfunction, not overuse. Whether this is true or not, I do not have near the knowledge to determine, however, Egoscue does make a compelling case. I think most people with pain type problems that have tried traditional physical therapy exercises should at the very least give his program of exercises a try. There is so much to potentially gain. I am not affiliated with him in any way. http://www.peteegoscue.com
. . . and in my opinion Pete Egosque is right. On some philophies on training Pete's ideas differ from mine but on on this we are in the same camp. Most repetitive motion injuries are not so much a function of achieving middle age as a function of having more 'play' time to allow deterioration of the origin of the problem. Truly your hands are designed perfectly to perform for your whole life. The alignment issues that create impingement on the nerves develop over time and the hands are eventually the symptom area. Check your alignment and refer to some of the earlier posts that talk about hand and shoulder and elbow injury. You will find many tools to measure your alignment to see if you are creating the problem in your posture and not by using your hands too much. I played classical for a few years in college and know the position your shoulders and neck are in to create your art. If you aren't doing some exercises that let your body look at a straighter, more appropriate posture for homosapiens, then you can expect a symptom to show up somewhere to remind you that your spine is supposed to stretch out. It is during your exercises (both strength and flexibility) that the body is reminded of it's true skeletal blueprint and allowed to recover from the frequency of activities that make your life rich. It's time to get to work so you can get back to having fun. Vik