I am by far an expert on this, but from what I have read there are several things that will shorten the life of a tire.
1. Lack of use ( like a old rubber band, pull it back and it will snap ) The tire has aged in one position, once it starts rolling what was on the bottom ( flatter area ) may not want to flex as much due to ageing.
2. How they are stored. If not at full pressure then they are more prone to blow/separate when put back in service at full pressure. ( again, the rubber band example )
3. Heat. As the rubber is baked it will lose the ability to flex and the steel belts will not be held as well to the rubber.
4. UV will dry out a tire. Wheel covers help, but black wheel covers will draw more heat.
5. Quality of the original tire. Some manufactures put inhibitors in the rubber to slow the aging process, some don't.
6. Tire overload. exceeding weight, or speed ratings break a tire down real fast.
I am sure there are other factors that effect tires, like the roughness of the roads, hitting curbs, etc. All damage to a tire is cumulative. So hitting several bad pot holes may not have caused visible damage, but the damage to the belts is there and has shortened the life of that tire.
So depending on the factors above, my guess is 3 to 5 years.