My psyducking 20-year-old Yellow cartridge still psyducking works, holy muk (I just checked) (Oh the simplicity, I totally forgot that Special was one stat in Gen 1)!
Although it no longer has my first playthrough on it as I've restarted a few times since completing it, the playthrough currently on it should still be my oldest. It's hard to say as I've restarted all my games except Silver at least a couple of times.
I haven't done a trade up since the mass trade up I did from my first Yellow playthrough to my one and only Silver playthrough.
Ironic how if I hadn't, my very first save file would still be around. My Silver battery died in 2007 taking my most beloved Pokemon with it including the only 3 level 100's I've ever had to date (not counting Snakewood), my Pigeot (one of the up-trades from Yellow), Dragonite and Furret. :'(