I recently did a car belonging to an elderly snow bird who lives here in AZ only 4 mos. out of the year. The car they drive while living here is a 2010 Lexus that looks brand new. It only has 18,000 miles on it and is garaged 100% of the time.
While cleaning the tires, I noticed light cracking. I looked at the date stamp and saw that they were the original Michelin's that came with the car.
The NHTSA recommends that tires, regardless of condition, be replaced after 8 years based on the federally required date stamp on the sidewall. After 8 years, the structural integrity of the tire has diminished to the point that it is no longer considered safe at any speed and is even more likely to fail when cornering or stopping short. This applies to compact spares as well.
There are a lot of 8+ year old used cars out there with a spare tire in the trunk that unsafe for use.
I brought this to the customer's attention. The next morning he called me from Discount Tire while he was waiting for his new tires to be installed to thank me for the heads up.