Like Ed Lee, I am 75 now and the weight of time is causing me to have severe mobility problems. I just now got home from playing a concert with a prestigeous concert band at a monster annual festival of the arts here in Grand Rapids. I have decided to drop out of three of the bands that I have played with this last few years and play with just one of them. I have no playing problems at this point in time, just the problem of getting arround.
If you are not having any ambulatory problems and you are enjoying the making of music, I can not visualise any reason to curtail your musical endeavors. The advice to step down to second or third part is very valid. The company of fellow band members is, in my opinion, a treasure without price.