Half life means that the nukes just kinda become not nukes after awhile.
No, half life is the amount of time it takes the element to decay halfway. For Uranium-238, that is ~4.5 Billion years. Uranoum-235 is ~700 Million years. Plutonium-239 is ~24,000 years.
The issue with old nukes is all the other components aging, not the nuclear material.