Just like all tires not being equal, so is the care that the tires receive. If a quality tire, such as yours, is well cared for a 6-10 yr life IMO is accurate. With fair amount of care 6 years, and excellent care closer to 10 yrs. Tire life can vary due to inflation practices, storage surfaces when not in use, parked in sun, parked/standing long term, hitting curbs and potholes, overloading, or too high of speed.
You know more how they have been treated, than anyone here. If you feel they've had excellent care, I'd trust them another year or two.
Jerry