An email from Angel R. in Glendale, Arizona, told me he smelled bull: His tire shop recommended he replace his tires with only 29,000 miles and lots of tread left on them. Bad news: That pricey recommendation could well be right.
Tire rot is caused by ambient heat, UV radiation (, ozone in the atmosphere, road salt and time. These factors are mostly worse if your car lives outside, but even garage queens can't sit on one set of tires forever.
Six to 10 years is about all a tire is good for, regardless of miles. Inspect the sidewalls for tiny cracks on the surface of the rubber. It will look like cracks in the glaze of a piece of pottery. Then inspect the face of the tire and look for cracks around the tread blocks. What you can't see is hardened rubber compound that is less grippy and potentially worse at shedding water to avoid hydroplaning.
Tires have a date of manufacture molded into the sidewall, usually in the format "XXXX." The first two digits are the week the tire was made and the second two are the year, since 2000. If your tires have a three digit code, like "XXX," it's time for new tires: Yours were made in the last century.
Tire date codes can be tricky to spot as they're sometimes part of a longer string of characters, but the code will always be numbers only and should always be the last four characters in a string.
Sidewall inspection and date of manufacture deciphering are, admittedly, rule of thumb information. Where's the data to scientifically tell you the condition of your tires? It doesn't exist outside a tire lab, butto gauge how tires age.