Forgive me for name-dropping but Mike Rutherford told me recently that he gets really annoyed when people ask him why he still records and tours, even though he is a multi-millionaire. As he pointed out - if you make music until you are rich and then stop, it was obviously just to make money that you made music in the first place, and no genuine musician does that.
So musicians carry on - because it's what they do. What else would a rock and roll band do except record and tour? It's their life-blood and they are still able to do it, so why on earh not/ the Stones always cite the old bluesmen who always carried on until they dropped, admittedly most of them still needed to earn a living, but if music is in your blood, that's what you do.
Intriguingly, the young hipsters of pop's earliest days are now the Stones' ages, we have never had 'old' pop stars until now, but as as has been pointed out, if they can, and they want to, they should. There is no cut-off at which point any musician is going to say - my best work is behind me now, so I'll toddle off to the allotment.
Long may they all continue!