Yes, one's health should be more important than selling out concerts. If you're not well enough to perform as you used to, you should either take a break or reevaluate how you perform. Rock stars should be able to retire as well as any other working citizen. There comes a point when your body can't do the same things as it used to.
Yes, there is an age where a person is too old to rock. However, that age is different for each person. An individual is too old when he can not withstand the vigorous physical activity involved with performing on stage, when he cannot handle the heat of the stage lights, when the schedule is too busy, or when they're just tired. An individual can determine that for themselves, or with help from family members or band members.
You are never too old to get your rock on. Even if you die on stage, you died doing what you loved. Rock and roll is an experience, a meeting of the soul through rhythmic expression. Who would not want to experience that. Keep rocking out Joe and others like you.
No matter how old you are, or in special cases, how disabled you may be, there will never be a limit to how long or how much of it you can do if your mind is really set after it and you have a passion for it and a burning desire to do it. That's one strong belief of mine.