I am not sure if the disks would still work but I've got a box full of Amiga games and most will have saved game data on them. The oldest will probably date back to around 1992.
Yeah, now i think about it, I still have my copy (in the most lieral sense) of Championship Manager Italia on the Amiga. I played so much of that i probably got to the 21/22 season. If i had the means i would give it a go and find out if the save is still intact.
I'm pretty sure Mario World was the first game I had with save functionality. I still have the cart, although I don't think my initial save file is on there.
The earliest I can think of that I'm sure I still have is Pokemon Red. I got it at (or very close to) launch and have booted it up in recent years, to find my Pokemon team still living a good life since 1999. Hope the save game is still there since last I checked. It's probably not the very earliest intact save I have, but an early and significant one nonetheless!