I don't think I have record of it any more, but the prev owners replaced two tires then four, a little over a year apart. When working on brakes, I noticed cracks along the tread grooves in several places on both of the older tires. My best guess is they were eight years old. I replaced all six that were on the ground,as well as the spare which had been unused its entire live, from manufacture in 2001 till I replaced it in 2015.
So, as you do your own tire inspections, be sure to look IN the tread as well as for separations at tread edges and checking in the sidewalls.