I've heard/read about how shot the shelf life is on tires, but I frequently run tires that're kinda old and I've never noticed any problems. The (original) tires on the S8 were made in 2000-2001 and they're just fine.
I'm not about to take the Jag's original 1985 tires on the track or anything, but for normal puttering around they're fine too (I have a set of newer tires for regular driving). I've read about collector cars that're driven on much older tires than that, and while they don't exactly handle great and shouldn't be subjected to extreme stress, the tires aren't blowing out or coming apart either.
But yeah, I've had cracking/bulging on tires that were only a few years old, and I replaced them immediately. If a tire has visible issues, I won't drive on it.
Snowtires *do* seem to go bad quickly...it's like they harden and won't work in low temps. Snows that looked perfect simply didn't drive right, so I recognize a short shelf life for those types of tires.