I guess the oldest games I ever played were Tetris and Super Mario Land on the old Game-Boy Classic. (They were both released at about the same time in the PAl regions. September 28, 1990 if Wikipedia is to be believed.)
Incidentally, those were also the first games I ever played.
Well over a decade later, and it's still going very much strong. Although I have become way, WAY more jaded than when I was... Damn, I forget what age I was back then. 5 or 6, maybe?
Although far more recently, I acquainted myself with the original Dooms.