From our website's entry on Memes, it's defined as "culturally transmitted information, or ideas and beliefs that can be spread from one organism, or group of organisms, to another. A key component to the meme concept is that the information is able to self-replicate, and in turn undergoes a type of natural selection, much like genes."
So what would be considered the oldest living, occurring, still-used meme? I'm placing my bets on "Bless You", a Northern phrase uttered immediately after someone sneezes. The origins of blessing someone after they sneezed seem to stretch as far back as 70 AD, but this is an inaccurate date as there is no precise circumstance as to how it arose. The real date of origin has to be around 590 AD. Who was the "memer" that started this trend of blessing people when they sneezed? Pope Gregory I. He gave orders for immediate prayer for divine intercession, one being to bless people when they sneezed. Sneezing was, at the time, a telltale sign of a big illness, the Plaque of 590AD. It became a staple of Northern culture to say this phrase after someone had sneezed by 750AD.
I think it would be interesting to know what the oldest surviving and still-used meme is (inb4 we've already done this), what are some other peoples' guesses?