In stasis, rubber "deteriorates" rather than "wears out" http://www.astm.org/Standards/D572.htm

Sunlight, temperature, pressure, the chemical composition of the air, compression, ... are other factors.

I have a low-mileage 1947 truck that sat in a (mostly heated) warehouse from ~1953 until 2004.

Most of rubber on it was brittle. Some crumbled when removed. Some of the bushings had a bit of elasticity left. The windshield gasket was brittle on the outside but had some flex left inside. The rear window gasket was OK - the truck was parked where sunlight hit the windshield but not the backlight.

Google "time capsule" and note the common recommendation that rubber not be included as it gives of sulfur as it deteriorates, which harms other things.
--
240 drivers & parts cars - Ohio

Yes, rubber "wears" out. Lots of things happen to it. If it's exposed to the sun, it happens faster, so trailer boaters like me put tire covers on our trailers when we're not towing for a period of time. But even if it's covered, or stored in dark garage or in your basement, mere exposure to the air is bad because of chemicals in it (ozone, etc.). The small cracks and crazing you see in old rubber is the result. And rubber can simply dry rot -- I think it's a kind of loss of chemicals in the rubber that keep it flexible. All in all, over a variable amount of time rubber loses its qualities. Think of your experience when you find an old rubber band in a desk draw -- instead of stretching, it just cracks.

And this is not including the affects of continued stress on rubber. Places where the rubber is resisting some mechanical force (bushings on accessory mounts, end links, etc.) are eventually going to see the rubber part's dimensions yield to those forces and change its shape, no longer serving it's original purpose(s).

Calpete,

I though you were in Cally somewheres by your name. I see you are in Florida.

Yeah, rubber deteriorates. Why you need to replace tires every six years as tire rubber oxidizes. The rubber boots on ball joints and outer tie rods become brittle and fail to seal the grease packed joint therein.

Ever take a new rubber band and see how supple, strong, and stretchy it is? You then discover it in a drawer maybe a year or so latter and it is dry, sort of brittle, and the like?

The rubber compound in tires, and in suspension bushing (a different material from tire rubber), does deteriorate over time. Ozone, oxygen, ultraviolet from sunlight, exposure to non-polar compounds like hydrocarbon based grease and oil, and heat and cold, all breakdown rubber.

Plastic formulation varies greatly. So, it can deteriorate to give off funky smells, discolor, weaken to become brittle or rubbery.

Suspension bushings are under minimal stress, but yes, some stress, just sittin' on a level flat surface doing nothing. Bushings are installed and assembled loosely. Usually, you place the car on a rack, like an alignment rack, so all bushing retaining hardware is torqued properly with the car at rest on a level surface (like an auto alignment rack). Some refer to this exercise as setting bushing pretension so that the suspension bushings and other parts are in as relaxed state when at rest in your level, flat parking surface.

Hope that helps.

Volvo "Buttermilk" MacDuff.
--
Buttermilk in the PacNW is just that much better.
(A little pepper helps, too!)