Tire age

Moparone said
10 years max, way less if you race or drive hard. Even though the tires still look new, they harden with age becoming unsafe. Try pressing your fingernail into an old tire tread versus new and you will notice a substantial difference especially on more modern tire compounds.

Good point, but......

I had 11 years on a set of Nitto 555 tires on my Charger. I was having some vibration issues and thought that maybe they were out of balance. America's Tire wouldn't rebalance them since they were over 10 years old.
The rears had 40-50% tread, the fronts were 80%+ with regular wear. They still looked great, no age cracks, no chunking or hard feel to them. The balance issue turned out to be a bad transmission mount. Two years later when I replaced them simply based on peer pressure, I got a look inside of them. They were perfect inside as well....13 year old tires and they still looked great.
It might help that it rarely gets below freezing here. The car sits in my shop when I'm not driving it.