NES battery games are supposed to have around a five year life. And of course anyone who collects old carts will find carts with dead batteries that need to be replaced.
Well, I sat down to play some NES tonight. On a whim, I checked my Zelda II cart that I received for Christmas 1988 along with my NES.
Below: my original Zelda II save game from 1989, still on the cart! Thirty-one years later!

So, what is your oldest save game?