Cans of corned beef from the First World War have been often found to be fit for consumption. Considering that's nearly a century ago, twenty years should be fine.
Some of the stocks in old air raid shelters and nuclear bunkers from the 50s and 60s have also been examined with similar results. The key factors are dry, stable environments and undamaged tins. If they get wet, they will eventually corrode. If there's anydamage, it again provides an entry for corrosion. Not sure how far back it goes, but 'tin' cans are anodised on the inside aren't they, separating the contents from the metal with a thin inactive layer. If they're dented, this layer gets damaged and any acids, alkalis, etc. in the contents causes corrosion on the inside, thus contaminating the food. Not a problem if they're consumed soon after the dent, but any length of time, the food would probaby spoil.
Glass is made out of sand. Cleaner, finer, purer sand makes better, clearer glass, but basic glass can be produced in makeshift kilns. It might not give you a clear view, but it would let light through and stop the wind. For making drinking vessels out of glass, you'd want someone who knew what they were doing and they would need to make a more specialised oven, but it's far from impossible. I saw some very nice 14thC Venetian glass goblets a couple of years back. Definitely require a craftsperson, though.
Oh, and Chris? I still preserve fruit and veg in glass jars here. It's a good way of keeping it for the winter without freezing it.