Properly stored powder will last quite a long time. But if that can of 21 year old powder was previously opened you have no way of knowing if that is 2400 or some other flake powder.
If you sprinkle it onto the lawn or flower beds, water the same day to help dissolve the power. The plants will thank you for the nitrogen.
Mike's suggestion of looking for a reddish tinge or dust is a good one too. Even if it doesn't smell bad, that discoloration means it should be disposed of.
BillCA in CA (Unfortunately)