I remember being puzzled back in the early 70's by the fact that my new young friends in Toronto were all using the expression "old fart" to refer to people over a certain age (50? or ?). I thought it was just some sort of arbitrary derogatory term, and not a very nice one.
Now I realize it was not arbitrary! There was a physiological basis to the expression. Oh the horror!!! I have become what they were referring to, in the smug arrogance of their youth (which was soon to vanish).
My only comfort is that they have no doubt all become "old farts" as well, assuming they are still alive...and given the foolishness of most of them regarding diet and the use of mind-altering substances, I figure the chances of them still being alive are getting slimmer all the time.