It appears that there is little data on this. Bridgestone - Firestone just announced a 10 year max, but BFS’s bulletin says the company “is not aware of technical data that supports a specific tire service life”.
The following if from TireRack:
The British Rubber Manufacturers Association (BRMA) recommended practice issued June, 2001, states "BRMA members strongly recommend that unused tyres should not be put into service if they are over six years old and that all tyres should be replaced ten years from the date of their manufacture."
Environmental conditions like exposure to sunlight and coastal climates, as well as poor storage and infrequent use, accelerate the aging process. In ideal conditions, a tyre may have a life expectancy that exceeds ten years from its date of manufacture. However, such conditions are rare. Aging may not exhibit any external indications and, since there is no non-destructive test to assess the serviceability of a tyre, even an inspection carried out by a tyre expert may not reveal the extent of any deterioration."
More recently, The Japan Automobile Tire Manufacturers Association (JATMA) recommended practice issued May, 2005, states "customers are encouraged to have their vehicle tires promptly inspected after five years of use to determine if the tires can continue to be used (recommends spare tires be inspected as well). Furthermore, even when the tires look usable, it is recommended that all tires (including spare tires) that were made more than ten years ago be replaced with new tires. Additionally, because in some cases automobile makers—based on the characteristics of the relevant vehicle—stipulate in the owner's manual the timing of tire inspection and replacement. Please read and confirm the content of the owner's manual."
Several European vehicle manufacturers of high performance sports cars, coupes and sedans identify that "under no circumstances should tires older than 6 years be used" in their vehicle owner's manual. However, it should be noted that European recommendations must include driving conditions that include roads like the German Autobahn, which allows vehicles to be legally driven at their top speeds for extended periods of time.
While American driving conditions don't include the high-speed challenges of the German Autobahn, the U.S. divisions of DaimlerChrysler and Ford Motor Company joined their European colleagues in 2005 by recommending the tires installed as Original Equipment be replaced after six years of service. (General Motors declined to offer a recommendation until a more scientific analysis of driving conditions and tire aging could be completed).
It's common for The Tire Rack to receive new tires direct from the manufacturer that are three to six months old. Since we rotate our inventory, most of the tires we ship are less than a year old. There are some occasions where we work with a tire manufacturer to help them clear out their warehouses when they discontinue a tire line. While this clearance frequently uncovers some new tires that are several years old, these tires are offered at a significant discount and will most likely be worn out within their useful lifespan.
It is also important to note that the more aggressive the tires' tread compound, the shorter its life. DOT-legal competition tires would have the shortest shelf live before the aging process would begin to diminish their performance. Sometimes the tires that were just right at the end of this season will be too aged to be fully effective the next year. Max Performance and Ultra High Performance tires would be next, and so on down the line.
Our experience has been that when properly cared for, most street tires have a useful life of between six to ten years. And while part of that time is spent as the tire travels from the manufacturing plant to the manufacturer's warehouse, to the retailer and to you, the remainder is the time it spends on your vehicle.