The answer to powder stability with age depends entirely on how the powder was stored and what the environment was, so there's really no single easy answer.
Real black powder is the most stable; it can be rendered unusable by direct contact with moisture, but if spread out and allowed to thoroughly dry out, then sifted, it will return to as-new condition.
There is anecdotal evidence that Pyrodex can degrade over a long period of time if exposed to local environments (changing temperature and/or humidity extremes), and it does not appear to respond to 'rehabilitation'. However, many people report no degradation, especially if stored properly.
My own experience with 777 is that it can be stored over at least 5 years without noticeable degradation; I keep it sealed in original containers, so I don't know if ambient exposure causes a problem over the long term, and I can't comment on whether it can be rehabilitated.