Oldest save? Probably on my Zelda II cart dating back from November 1989. Provided it still works, next up would be my save from Dragon Warrior dating back from May 1990. I remember coming home from Exams in school that June as fast as I could, just so I could explore more of the Dragonlord's Castle. Just a little further to see what lied beyond...
I probably have my FFIV save on SNES ranking up there being from late 1991 or so. Nearly maxed out my characters with that one.
Oldest PS1 save would be from March 1996, with the original RE. PS2 save would be December 2000 with RRV. PS3 would be November 2006 with RR7.
Course I could look into my father's collection of ST and Amiga games and find say... my King's Quest III save from 1986 on the smaller floppy disks...
It's so wierd looking at old saves, now that I think on it. It's like a timestamp, showing everything frozen in time. In some cases, like my DQVII save on PS1, It's almost another language since I have since forgotten the nuances of that particular game as opposed to when I was hardcore playing it at the time.