Tread gets the attention because it is visible. The tire's manufacturing date is printed on the sidewall, and it does not wear off with the miles. A tire can look full and still be old. Rubber that has sat for years can crack, harden, and lose grip in ways a tread gauge will not show. Before a trip, a purchase, or a decision to keep a set, read the age first.
Find the code
Look for the letters DOT on the sidewall. The code follows those letters. The last group of four characters is the manufacturing date. The first two of those characters give the week of the year the tire was made, and the last two give the year. Older tires carry a shorter code with a different layout, so ask the shop to confirm the date rather than guessing from a partial read. Check both sides of every tire, the spare, and any trailer tires, because the codes are not always the same. Fleet and rental vehicles sometimes carry tires of mixed ages on one axle.
Age is not the same as tread
Tread depth tells you how much rubber remains. Age tells you what the rubber has been through while it sat. Look at the sidewall and the grooves for cracks, a bulge, or a surface that looks dried out. Ask whether the vehicle was parked outdoors in strong sun for long stretches. The tread wear indicator bars molded into the grooves show when the tread has reached its minimum depth. They do not show age, so a tire can pass the bar check and still fail the age check.
What to check and what to ask
Open the owner's manual or read the placard on the door frame. Note the tire size, the load rating, and any guidance the maker gives on tire age or replacement. Keep that page with the codes you record.
When a tire shop mounts or rotates your tires, ask them to write the date codes on the invoice. If the invoice shows the codes, you have a record to compare against later.
If you are buying a used vehicle, do not assume a set is recent because the brand is familiar or the tread looks good. Read the codes yourself before the sale is final. Ask the seller whether the tires were on the vehicle when it was bought, and whether they were replaced at any point with stock that had been sitting.
When a shop recommends replacement, ask which tire they are recommending and why. Ask them to point to the cracks, the bulges, or the age on the sidewall. A specific answer you can check against the tire is more useful than a general one.
Comparing a replacement
Compare the new tire against the size, load rating, and speed rating printed on the placard and the existing tire. Confirm that the date code on the new tire is recent before it is mounted. A tire that has been stored for a long time is still old, even if it has never been driven.
Keep the record
Write the date codes in the service log or keep the shop invoice with the vehicle papers. When the question comes up at a sale, an inspection, or a trip, the record answers it without guesswork.