Says the folks that sell the new tires...Another thing to consider is that tires do have a shelf life of ~10 years. That includes your spare tire. If your spare has been sitting around unused, its quality may be questionable when you need it. That's one reason why I do a 5-tire rotation.
Guide to How Long a Tire Lasts
How long do tires last? Learn more about how long tires should last, the factors to consider, and how to extend the life of your tires.www.bridgestonetire.com