Silvering is usually more of an application problem than a decal aging problem. The lack of adhesion might be an age problem but can also be due to other things. The biggest age problem is cracking- that is almost always due to age. I have generally found decals of recent kits not to have an aging problem, so I would assume decals last at least for several years. By the time they get about ten years old or older I get worried.
Lately, in fact, I always scan decals, so I can try to re-create them if things go wrong, before I soak them.
Yellowing, another old-age problem, can often be solved by bleaching in sunlight, though I always worry that that treatment may make them more likely to crack.