Rubber compounds in tires oxidize and harden over time, losing grip characteristics even if the tread looks new. Most manufacturers recommend replacement at 6–10 years regardless of tread depth.
The DOT date code on the sidewall tells you the manufacturing date. Look for 'DOT' followed by characters ending in a four-digit number — '3221' means the 32nd week of 2021. The tire is over 4 years old in 2026.
Spare tires are commonly forgotten. A 15-year-old spare that's never been used is structurally compromised and may fail under regular driving load when finally deployed.
Storage matters. Tires stored in direct sunlight, near ozone-generating equipment (electric motors, refrigerators), or in extreme heat age faster. UV light is particularly destructive.
Crash data shows aged tires cause a disproportionate share of single-vehicle highway crashes, especially in summer when heat amplifies the brittleness.