Why solid-fuel missiles still have a shelf life

Does solid rocket fuel degrade over time?

Yes. Solid propellant degrades chemically over time, helping determine the rocket's shelf life.

Solid propellant degrades chemically over time, which helps determine a rocket's shelf life. The rocket is the propulsion component of the weapon. Adding the guidance package and warhead produces the complete missile system. Keeping those terms distinct helps explain why a discussion of missile aging can focus specifically on its rocket fuel.