Rock and roll is still attributed to youth. Something you do when you're young. Like punk or trying out new drugs. Hence the jokes when old people play rock and roll.
They still have a point though. Because the songs Rolling Stones play in concert is still those songs they wrote when they were young.
In a way to see the Rolling Stones now is like chasing your own (lost) youth. Nothing wrong with that though. The Rolling Stones is still an aged symbol for eternal youth.
Like an ongoing Peter Pan syndrome.