Most all my vehicles' tires age out while there's still plenty of tread - so far this year I've spent $2,000 on tires.
One set was actually worn, being the daily driver, but the tires on my Dually - which towed loaded car trailers last year - I couldn't recall how old they were.
They looked fine, and had more than 50% tread, so I went through my file on the truck.
Turned out they were 21 years old. I figured that's as far as I want to push my luck; I really like that truck, the trailer, and my other vehicles...