I have a 12 year old brand new tire for a spare on my truck. I should probably replace it even though it's never been on the ground. OTOH it has never been exposed to sunlight.
I can't add it to the rotation now because I changed tire sizes. (also the wheel probably doesn't match -- even if it once did) If I'd thought it through I would have done a 5-tire rotation from the beginning and I would have gotten an extra year or two from that first set.
NHTSA says the absolute life expectancy of a tire is 10 years. Maybe I'll change it at 20...
No you won't.
I recently mounted up a set of radial tires that were just about 10 years old but only about 40% worn and put them on a new-to-me used Cherokee I recently bought. I stopped by a friend's house a week or so later to chat. He's a VW shop foreman, and of German ancestry, so he is "picky." He started looking at my tires and said, "You're not planning on driving home, are you?"
Of course, I was ... so I asked why. And he nicely pointed out that the sidewalls all had a neat crack, running all the way around, about a quarter of an inch out from the edge of the wheel. I didn't see it when I had the tires mounted, and if it had been there I'm sure I would have seen at least one of them. So I had to bite the bullet and spring for a set of new tires.
I have another pair of tires that came off an older Cherokee that I saved because they had excellent tread left. Can't use 'em -- the sidewalls are all cracked just from sitting.
Turns out it's actually better for the tires to drive on them. The flexing keeps the rubber "alive" (or something like that -- it has to do with distributing some chemical through the rubber.) It also helps to periodically spray the sidewalls with "tire foam." It sort of cleans, a little, but it also has silicone in it that helps preserve the rubber. I spray the outer sidewalls at least every two weeks, and I spray the inside sidewall when I rotate the tires.
