There's a post somewhere above referring to a 69 year old instructor. My PPL instructor is 81 years old! He started his career with the military, then moved through BA's predecessors and BA itself, then on to Singapore for five years post his BA retirement. Then he took up full time instructing and examining - has been ever since. He's sharp as anything and has provided many many students with years worth of annecdotes from which we've all learned to be sensible safe pilots (well, mostly!)
I know instructing is the current route to the airlines for many young pilots, but if more retired pilots took it up they'd be able to continue enjoying flying at someone else's expense, students would benefit from their years of experience and the airlines may be forced to take a more serious approach to ensuring an ongoing pool of skilled new starters.
HH