One of the guys I ride with has a quote that "We do not stop playing because we are old, we become old because we stop playing". And I think the point of it is that "old" has a lot to do with attitude.

On the other hand, with the proper attitude, it's okay to be "old", too.

When my grandparents were aging, they went to lots of funerals. My mom was one of 8 kids. But here a while back, one of my uncles died, and none of those 8 kids or their spouses were there for the funeral. So to me, you know you're "old" when you're too old to go to funerals! Then again, maybe I'm judging them too harshly. They might have all been out rock climbing or running marathons and weren't able to make the funeral because of that.