If we include RPGs, I am playing through the Classic Traveller Adventures from 1980.
But to rules, the oldest I would consider playing are Ancient Warfare. It is funny you should mention old rules as I have just got into an email conversation about Ancient Warfare.
Older than that is I really want to play Tractics again but I have a feeling I never will, mainly as every time I look at them I wonder if I will enjoy it as mush as I used to.
Armati II, DBM 2.2 and the Irregular Miniatures Ancients rules are about the oldest sets that realistically I will be playing every now and then.
So nothing too old really.
p.s. for those that are interested, here is a link to my review of Ancient Warfare: