To be honest, an old/out of date bag of plaster can be a bonus if you're patching up or you have a small wall etc, as it'll go off quite a bit quicker, but if you're a bit of a novice, it could cause a problem. If you're unsure, buy a new bag.
Obviously the shelf life on the bag is for a reason and to cover the manufacturers against any come backs by using out of date gear. You would probably find that it would go off quicker than normal, If you are doing smallish areas you could throw some in with your new stuff to send it off quicker!! But I never said that ok!!!!
Thanks guys. It did go off in no time, about 15 minutes, but that was no problem as it was only to smooth of a bit around a couple of sockets. What is less, is that after 24 hours it is still not very hard i.e. I can scratch it easily and quite deep with my nails.
Nothing a bit of pva won't solve, but I was curious to see if the strength was expected to be less.