Well, there’s “still play”, and then there’s “still play”. The oldest published RPG I have played recently is probably The Shadow of Yesterday (we’re playing it now). I also regularly return to Primetime Adventures, though it’s been a couple years since we last played it. And Primetime Adventures is pretty much my go-to game when I don’t have some other game specifically in mind, so I’m sure I’ll play it again some time soon.
And then there are several older games that I would play at the drop of a hat, but it’s been many years since I last have: Ars Magica, Werewolf: the Apocalypse, Underground, The Babylon Project, Everway, Fading Suns, Deadlands (the original), or Over the Edge. I still buy all the Ars Magica supplements as they come out, and try to find time to read them.
The oldest published game I actually have played semi-recently is Rolemaster: I played for a bit about 4 years ago with a group that is still playing the original edition, 30 years later. But I don’t really think it’s fair to say that I “still” play or read it—I haven’t touched it since then, didn’t read any of the books at the time, and before that group I had last looked at a Rolemaster book in about ’85.
The only games older than The Shadow of Yesterday that I’ve both recently played and intend to play again in the future would be my own Four Colors al Fresco, created in 1999, released as a free beta PDF around 2004, but not yet properly published. It will be this fall/winter, if I can make the time between school and work.