I brewed once using >1 year old centennial pellets. I used the hop age calculator in Beersmith to approximate the alpha acid degradation to adjust the IBUs. Result: way over-shot my bitterness. IMO, properly stored pellets degrade very slowly, it's hard to say when they really have reached a point where your bitterness will be affected. I've heard on the brewing network that year old pellets showed almost no degrading of alpha-acid. I think it was the IBU formula Brew Strong show.
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