Automotive Smog Check Tips
Is The Age of a Tire Important ?
Youbetcha !..The National Highway Traffic Safety Administration
(NHTSA) recommends that any Tire SIX YEARS OLD or older
should be replaced , regardless of Tire depth. This means that tires
that look almost like new but are 6 years or older should be replaced
because the NHTSA determined that age, not tread depth, was a
major contributor in tire failures.
Automotive Principles Technology - James D. Halderman (1999 Edition)