Yes, flares do become unstable with age, as can be testified by one of the Skippers who had an old flare blow up in his hands!
However, I have unofficially heard it said that it should be safe to keep the flares for upto a year after their expiry date as a spare, but these should not be considered as a main flare because their peformance/effectiveness will have deteriated.
But how can you advocate this on a forum? An expiry date is an expiry date, I guess. Owner beware! If the flares have not been properly kept this could be bad advice.
BF