Don't know the exact year it was made, but found a bunch of mixed .22 loose rounds that had fallen down inside a hollow false mantel. CBs, shorts, longs, LRs, all mixed with bills and coins going back to the 1920s. Everything worked.
I've found older stuff but I didn't fire it. I picked up some boxes of .38 S&W and .41rf in some auction lot boxes, which didn't fit anything I had. I also found three rounds of .45-75 in a chopped-up '76 Winchester. These went along with what remained of the gun. I've found other old, loaded guns, but none that I was willing to fire as-found.
The oldest non-surplus ammo I have right now is one last box of Federal Hi-Power .22 Longs, probably from around 1960. I used up the other three boxes about 11 years ago. Again, it all worked, no misfires.