Everyone knows to look at the car tires now and then to see how the tread depth is holding up. And if the tread is thin, the tires get replaced for safety. But what most people don't know is just as important.
The little secret is that tires more than six years old, regardless of tread depth, present a large safety hazard. When people call me about single car wrecks, like when someone runs off the road and hits a tree, I usually wonder about the age of the tires. They degrade with age. That means they get weaker. Tires have a limited service life. The internal structures degrade, reducing adhesion between the belts. That in turn leads to tread separation.
If you remember the problem with Firestone tires coming apart several years ago, that was an example of tread separation caused by improper chemical adhesives. The same happens with age degradation. Age degradation is invisible, so it can sneak up on people, and it occurs regardless of tread use and wear. The degradation of a spare in the trunk is just as fast as the breakdown occurring in tires touching the road.
When you buy a tire, you may expect it to hold together until the tread wears out. But that's not what research shows. You can't take a stored vehicle out and drive it safely if the tires are six years old, no matter the amount of tread on the tires. And you can't put that old, but fully-treaded, spare tire on and drive safely until you get a flat tire fixed. If you have to drive on it, go slower and be cautious.
By the same token, a tire that has been stored for some time in a tire wholesaler's or retailer's warehouse might be brand new to you when you buy it, but very old in reality. That tire is dangerous. If a tire separates and causes injury, you can determine its age from the 11-digit code molded on the tire, but it's better to check that age before a wreck.