Its too old to ride when your body says so or you don't enjoy it anymore.

I'm 58 and can out ride most of the guys in their 30's that I work with. I ride just about everyday I enter cyclocross races in the fall because its fun to me. I figure I can spend my money at the bike store or the DR's, the bike store is a lot more fun. I'll be retiring when I hit 60 just because my job is moving about three hours north in two years or so I don't think I want to go. Wife and I will downsize and sell our house move to a small ranch (I like my own land not a big condo fan myself).

But ride heck as long as I can, helps me wake up and see the grass from the right side every morning.