iv noticed my last couple of hoppy beers using the 2013 hops were'nt great, these hops were from the usual sources which iv heard are nitrogen flushed, foil packed and stored cold, by the time they come into my hands i throw them in the freezer and keep them there till im ready to use. is this normal with older hops, i doubt its a recipe or process issue since i done nothing different.
They lose their potency. Use the hop age tool in beersmith to adjust the amount you use. 2013 harvest will have dropped by 4% AA if stored in the freezer in a zip lock bag.
I've brewed using nearly 3 year old hops and results seemed fine.
As a rule of thumb I usually increase the amount of any hop by 10% for every year. For example, 2013 hops will get 10% extra, 2012 get 20% extra, etc. It's not as accurate as Beersmith, but works fine as a quick fix if you're finding hop aroma and flavour are less than expected.
I'd never bother using old hops, just toss them and shell out the few quid for a fresh pack. I do like my hoppy APAs though so fresh hops are a must I find.
A lot of people have a few kgs of 2013 pellets still. I wouldnt go chucking em. They're totally fine once they've been stored correctky, just adjust your recipe for the age.