One of the ingredients tire manufacturers use in their rubber making process is a chemical compound called AO, (anti-oxidents, and there are several different types). The purpose of AO is to make the rubber more resistant to environmental factors such as ozone, sunlight, etc. As the tire ages, so does the effectiveness of the AO, similar to chlorine in a swimming pool getting used up due to exposure of sunlight. The less exposure your tires have to harsh conditions extends the useful life of the AO. Because manufacturers can't control how end users treat their tires, they set a very conservative timeline on tire life because after all, these products are hauling people and lives are at stake. 8-10 years is the number I always heard too. I just retired from Michelin, and we made the rubber for most of the Michelin plants in the U.S. Of course I'm not a spokesmen for them in any way, and sadly I have Goodyears on my toy hauler, I just know how the rubber is made.