The general rule of thumb is a tire is good for six years from date of manufacture. That assumes no special storage. After so much time the tread and sidewall compounds start to break down...even with no mileage or being mounted.
With the interior of the tires exposed to the atmosphere airborne elements can start the deterioration from the inside out. Rayon, while a strong fiber, is water soluble and will begin to come apart once it absorbs moisture. It's a good thing few, if any tires, use rayon anymore.
That being said, if stored in a temperature and humidity controlled environment, tires can last a long time, but the industry standard still stands since they cannot be responsible for how old tires hold up with any kind of storage.
I have Coopers on several cars and can say I've had excellent service from them. On my SUV, the Cooper Discoverer tires weren't worn out after about 60k miles, but I could feel them losing their grip in the wet, plus they began riding hard. The tread compound had changed with age and heat, so for safety's sake I replaced them...with another set of Cooper Discoverers.