They will still age and degrade, but the cold temperatures and lack of oxygen due to their packaging prolong the process. It's kind of like a sealed or open package in your fridge and freezer. The better sealed and the colder stored, the longer whatever it is will last...but only up to a point.
Depending on the seed type it may only be a relatively short time, say a dozen or two years, or it may be decades or much much longer for a more hardy seed. Whatever that time limit is, seeds can be rotated out, planted, and hopefully new generations grown to be refrozen perpetuating the cycle.