https://blogs.scientificamerican.com/observations/an-unanswered-question-at-the-heart-of-americas-nuclear-arsenal/

The United States has an arsenal of some 3,800 nuclear weapons, about half of which are deployed, with the rest in storage. The Department of Energy’s National Nuclear Security Agency (NNSA) and the nuclear weapons laboratories it oversees are concerned that the performance of the weapons will degrade over time. Most of the components can be—and are being—replaced with new versions, so the main concern is the behavior of aging plutonium “pits” at the core of all U.S. weapons.

Plutonium essentially doesn’t exist in nature but is produced in nuclear reactors. It was first produced during World War II, and the U.S. government didn’t care about pit aging because it constantly replaced pits with newly built ones as it upgraded its arsenal during the Cold War.  So, no one in the weapons labs knew what the lifetime of a pit was. Instead, the NNSA simply assumed it was 45–60 years.