Twenty-year-old ammo is still as fresh as the morning breeze. I've got ammo that old in my supply and it's still "new." Of course, I haven't subjected it to extreme heat or moisture, but heat, I think, is the greatest enemy of almost anything. And moisture comes along second. Still, most factory ammo is sealed fairly well against moisture. There are people who have 20-year old primers and powder and they work just fine.
I've got some reloaded ammo that someone gave me with my ancient reloading equipment and I won't shoot it because I don't know who loaded it or anything about it. I also don't know how to get rid of it, so if anyone knows, please say so. I don't feel comfortable just throwing it away.