The material simply breaks down over time, and manufacturers of ropes, slings, harnesses, and helmets suggest around five years as a conservative product life. Metals, on the other hand, show very little decline over time, other than superficial exfoliation of aluminum or oxidation of steel. Thus, if you are going to use that old Friend, at least replace the sling. The above guideline pertains to gear left unused and in the dark. Obviously, gear that has been abraded or banged around will need much more frequent replacement.
Abrasion is easy to inspect.
Tougher to see, and thus more insidious, is the destruction caused by UV radiation. Sunlight is very harmful to most synthetics. Ropes left out for long periods of time should be relegated to an afterlife as a rug or a clothesline. I was especially convinced of this when a partner of mine reached up to clip a faded one-inch sling attached to an old piton on a seldom done climb. Wild Country and other manufacturers continue to refine and improve their product line.