It depends on the packaging used, some older plastics used breathe oxygen very slowly over time and chemistry oxidises, this happens with some liquids as well where high quality high density polythenes etc aren't used.
As Ian says it all depends on the packaging. I believe that Kodak's is the best in this respect. I wish they still used metal cans. I have some film developer that is government surplus from WWII which looks like it was made yesterday! It is sealed under nitrogen.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.