1984 - Commodore 64 - Thats Old School days for me
Well my definition of oldschool stuff isn't necessarily '2 decades plus'. It actually varies. I kinda consider classic PS2 games 'oldschool' when comparing them to nowadays' gaming standards. Operation Surma is from 2003, that's almost a decade ago, which is roughly half my life. I guess for me it's bound to be old school.
If we're talking about 'real' oldschool games, I'd name Doom 2 (1993) as a good example from my point of view. I used to play it at the age of 5. It was my first PC game aside NFS1. I still play Doom 2 these days, last time I played NFS1 was 4 years ago.