The absolute oldest? Probably something for my NES. I don't fire up the physical hardware much these days (it's in desperate need of a new 72 pin connector after two decades and at least two owners), but I occasionally fire up an emulator and go to town. I've also got some really old DOS games, but even the oldest of those were most likely released in the mid 80's.
As for the oldest that I play on a regular basis? Probably something released somewhere around 2004. I've got a fair few games from that time period that I still fire up all the freakin' time, wheras my older games, while they still get a fair amount of play, I play more in situations where I'm like "hey, remember that game with..." and then I go play it. I've got literally hundreds of games in my collection, with actual hardware going from the NES to the present, and emulated releases (including official releases; I've got a couple of those collections of old games) going back to Atari's old arcade offerings. Heck, I've even got one of those plug and play controllers that's basically an Intellivision with some hardwired games. What I'm saying here is, while I don't play the old games as often as I do the new games, it's because only a small percentage of my collection can be in any sort of rotation at any given time. Since I enjoy online multiplayer so much, and since indie games have been hitting the sweet spot with older genres lately (If you haven't bought the Voxatron bundle, do it, do it now.), I don't feel as much need to go through the hassle of either hooking up the old hardware or getting an emulator set up -- especially since most of my collection is at home, an hour's drive away from my dorm. I've pretty much got the games I have through Digital Distribution (not just Steam; Impulse and GoG, too), the few boxed games I brought with me, my PS2 and PSX games, and my DS and GBA games. The NES, Genesis, Gameboy (Color), and the majority of my older PC games are at home.