I do not think one has to be too old to rock, but one can be. Rock comes from within.
And by that I do not mean it as condescension. A person can still be a creative musician but no longer be a rock and roller. Life has simply led them to other things. To me there is nothing wrong with that. Others, however, take their life experience and use it to make better RnR. Nothing wrong with that either! :)
What is to me sad, however, is someone who has grown past their rockin' years who feels the need to pretend that they've not. And it is especially sad (to me) if they are forced to play the same old song(s) that they played in their younger days if/when they no longer feel them.
When my `60s group The Abstracts recently had an album released of material we had recorded back in the `60s we had some interest shown in out doing a European tour. But in the final analysis we realized that while all of the living members of the band still "rocked" we had individually moved on style-wise. Me to naked acoustic rock and blues. To redo our old songs once or twice would be a hoot. But to be forced to do it night after night would not. No, I'd rather be in the audience then that!
As one member of the band put it. `Can you imagine playing one of out more popular songs for the twelve millionth time, for instance on a cruise ship for wishing-they-were-young retirees?' Ach!
But the odd thing is that this person still regularly plays with a band for the pure pleasure of it, doing much of the general `60s repertoire. And I dig back deeper -- often into the Chess catalog from the fifties.
I say: Be who you are, not who you were. And enjoy it! :D
-don