Info from a couple of web sites:
Consumer Reports:
"Tires do age and become unusable after time regardless if they were in service or not. Some auto manufacturers like Audi, BMW, Ford, and Volkswagen recommend replacing tires after six years, and some tire manufacturers agree that any tire ten years or older should be replaced regardless of tread wear."
Tire Rack:
"Tires are manufactured by bonding rubber to fabric plies and steel cords. And despite the anti-aging ingredients mixed into the rubber compounds, there is a realization that tires are perishable, as well as a growing awareness that some tires will actually age out before their treads will wear out." " Our experience has been that when properly stored and cared for, most street tires have a useful life in service of between six to ten years."