Oldest that I can possibly remember... Challenge of the Ancient Empires - a Learning Company game, but fun nontheless - I think.

That would be pretty old, too. Let's see... well, one internet site lists it as '91, so it can't have been the first. It lists Reader Rabbit as older, so maybe I played that before. Maybe Math Blaster, too. It's difficult to remember which game was oldest that I played. I was 6 in 1990, so that was some time ago. Wait... Operation Neptune, another Learning Company game, is listed as 90', so maybe that is one of the first. Wait, Treasure Mountain is earlier yet. '89. Wait... Outnumbered. Outnumbered was probably the first of those game that I played. That MUST have been amongst the first that I ever played - I think we still had it on B size floppies.

I honestly can't think of anything much older than that. I probably first played it in 1990 or so when I was 6, and my memory even of that time is... fuzzy.
No! Wait... older memories yet are stirring. I was playing a very, very old golf game. I remember that. There was also Aldo. Let's see, some game that I cannot remember the name to. A Tetris-like game called Beyond. I think there was a pac-man in the group, too. These were all dos programs. Before Windows 3.1 came out I think. I still remember as a kid playing that golf game when my parents were out and my grandmother was babysitting, and accidentially changing the directiory, and sitting there for an hour or two not knowing how to get back.
Oh, by the way, regarding the Keen games... I remember playing those for MANY hours. Keen 4, especially, and Keen Dreams.