There really shouldn't be a mandatory retirement age for rock stars (or anybody else, for that matter). Anybody should be allowed to do whatever they want -- as long as they feel they can still contribute -- until they decide it's time to hang up the instrument forever. Case in point:as part of The Who, Roger Daltrey sang the line "I hope I die before I get old." While that was an appropriate expression of youthful defiance back in 1965, it's totally different today...Daltrey is nearing 70 and his band's rhythm section has already left this mortal plane.
Does Mick Jagger look silly sashaying across the stage at 68? Sure he does. But if people are still willing to pay $150 to see him, I don't see the problem in letting him do it.