Ha! Ha! Ha!
A post on MetaFilter today cites an article in Smithsonian Magazine from February 2008.
“Found: the oldest joke in the world,” read the headline in the Sunday Times of London on June 29, 1997. Inscribed on a roll of papyrus, the hoary jape could be translated as a riddle: “How do you entertain a bored pharaoh? You sail a boatload of young women dressed only in fishing nets down the Nile and urge the pharaoh to go catch a fish.“
Uh… I don’t get it?
I guess you had to be there (in 2600 b.c., when King Snefru received this wink-wink nudge-nudge advice from the court magician Djadjamankh.) Or perhaps it was funnier in the original hieroglyphic. (Asp jackal ibis? Wiggly line, ankh, feather!)
Anyway, MetaFilter goes on to mention a bunch of other websites with crusty, old, historic jokes from bygone days.
So, on that note, I should like to share with you my personal oldest joke, a hilarious riddle I composed at the age of five:
Why did the chicken cross the road only halfway?
Because he was a crossing guard.
Yep. Made it up myself.

that’s almost as good as my mom’s favorite joke:
what is small, invisible, and smells bad?
worm farts.
I hope she made that up.
Don’t you get it? Nets! Nets! And women in ’em! An a couple of ’em were twins (that’s kind of an inflection thing, actually)