Guy in my club brought in some porter that was in the attic of a friend (in FL - where your attic gets to 120 deg) that was brewed by 2 college students in the late 80's. It was in a a green Grolsch style bottle. We all tried some. It wasn't terrible, had some serious sherry notes going on from the heat and oxidation, but overall they must have had good sanitation practices because it was still sorta drinkable. I would say that oxidation kills most older homebrew. That being said, if the zombieapocolypse came and I needed a drink, I'd still drink that 20 year old brew.