Hmmm I'm not sure where it comes from but my grandpa calls himself an old fart so I do it too xD. We kinda have a joking relationship though XD he calls me rat/brat all the time. when I use it though it's not out of disrespect, I was raised by my grandparents so I think 'old' people are just awesome :P.

LMAO!!
where'd young punks come from?
LMAO!1 thats gonna be my new word(:

well i don't call all old people old farts but the mean ones can be i mean me n my friends like to sit on the golf course behind our houses. And theres this old guy that likes to bug us for just sitting there now he's an old fart >.<