How old is too old?

I think it's when you look back at your past work, do your best to give yourself an un-biased review, and see that your older work is much better than anything your doing now. At that point you should probably retire after giving yourself one more chance, in my opinion, or try going in a new direction.
 
Yeah. If you've been doing it for 30 years, it may not be so comforting if it's much better than your new stuff, since that might be telling you it's time to retire, though.

EDIT: 30 years is just an example, it can be just 10 years, or even 5 for some people.