When I go to visit my mother in an assisted living facility and see people that live there that appear to be younger than I.
Also when I scan the obituary page and see that more than 50% are younger than me.
... when you can let a Hall of Fame fart in a public place (like a grocery store, for instance) and act like not a damn thing is wrong. That's when you know you've achieved "Old ***" status.
BTW, I'm 41 and don't make noises getting up or down, in & out of a car, etc. Just get your *** off the couch & away from behind the PC and do something outdoors. Of course, being addicted to mountain biking helps.
When you do something strenuous, and you think, "my legs, arm, back, whatever...are really gonna be sore tomorrow." But they aren't sore the next day...they are sore the day after that. That's when you know you are getting old. What causes that? Doesn't make sense.