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.
I think a 50 year old "lost" bomb would be a radiological hazard at most. Nuclear weapons require upkeep.
After 50 years, only 6% of tritium remains from the initial 100%. Any weapon using tritium (boosted fission bombs and hydrogen bombs) would likely fail to work.
Chemical explosives that initiate nuclear detonation also degrade. Successful weapon function requires an extremely precise initial chemical explosion, not likely to be possible with a 50 year old bomb.
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.