[MichaelCollins]

I don't think the basic Pu-239 / U-235 cores themselves degrade, those isotopes have extremely long half lives, but other components of the bombs certainly do.

Particularly, the tritium used in boosted nuclear weapons has a half life of just over 12 years. Furthermore all Pu-239 cores have some Pu-240 contamination that undergoes spontaneous fission, the neutron flux from which probably degrades other components of the bomb over time.

[kayodelycaon]

Early weapons had a lot impurities in their fissionable materials, which significantly reduces shelf life.