You know who should stop, nobody that's who.
I think you get out there and do it until either you can't do it, or no one wants to see you do it. Morrissey may have aged but people still line up to see him and he's lucky enough to make a living singing his songs. I haven't bought a Morrissey album since I was in college but I don't care if people want to listen. Besides what else is Morrissey going to do?
I like a ton of artists whose best days are miles behind them but I respect the fact that they get out there and do it.