BA,
This has been answered here many times.
6 years.
Here's one reference:
https://www.discounttire.com/learn/tire-aging
If your tires get you to the tire store safely, buy a lottery ticket!
On the other hand, if you have a terminal illness that you can't live with any more, go out and stand on the throttle...
Sorry for the "forceful" answer; I was a tire engineer in another lifetime and often find folks who think that aged and decrepit tires "look" OK, so they must be good
Tires are made of rubber. Rubber ages inside the tire plies also - where it can't be seen. If you cut the tire apart you'll see slightly delaminating plies. Failure points that can't be seen from outside the tire.
Not sure why folks are reluctant to spend a few bucks on a new set of tires. Often they'll spend lots more on the car for other reasons without much hesitation. Tire reluctance? Who knows?
Good luck,
Tom