I think one steps into the realm of OLD FARTDOM when he can no longer get out of a recliner without making a grunting or groaning noise. That happened a little early to me because I spent my twenties racing bikes ( motocross and ice racing). I did some terrible things to my skeleton back then. Wuttablast! One other clue is that you can read an entire National Dragster while sitting on the can, and you don't even notice that the toilet seat put your legs to sleep. THAT, my friends, is an old fart.
The older I get, the smarter Dad was.