Old tires

In the United Sates, a yound man and his friends died in a car accident.
The police said that they weren't drunk, but a tire of their car was too old.
His father couldn't believe it, because he knwe that their tire was still good.
However, he didn't knew that the tire looked still new, the inside of the tire was old.
Tire can cause traffic accident when it is old, and those who are older than 6 years are pretty dangerous.
When his father bought the tire at an auto shop, it was already 4 years old.
The shop can still sell those tire because the tire don't have expire date.
So when the young man and his friends was driving a car, the tire was actually 9 years old.
We should be really careful when we buy tires.