Tires only last 5~7 years, maybe 10 if you are real lucky. Tire covers don't help. trying to wear them out actually helps. The problem according the tire professionals I used to work with is that the adhesives that hold the layers together just age out. Apparently, there are parts of all of this that get circulated when they are moved, but no guarantees there either. The best bet I replace a pair that are close to 7 years old and keep replacing a pair at a time for those three years. In three years, start again.
Matt