2hats
Dust.
Topols use solid motors but SS18 are hypergolic. Russian SLBMs use hypergolics in at least one, some all, stages.The Topols are mobile and can be launched from anywhere. The submarines are also permanently out and about with live nukes (like the UK ones). Not sure how the Russian silos work but the US ones are in a permanent state of readiness. They were designed with solid fuel tanks to minimise maintenance. Warheads need to be kept 'fresh' but it's a six year cycle and stuff should be being refreshed and replaced on a regular basis.
Some elements of warhead designs don't age well, whilst others are probably stable for decades. The physics package service lifetime very likely depends on how highly optimised it is for yield-to-mass (note: the Russian missiles tend to have the greater throw weights).