There are many reasons why tires should be replaced regularly. Here are some general recommendations based on currently available information.
The auto manufacturer's owner's manual should have specific instructions covering tire inspections & replacement considerations based on the vehicle manufacturer's understanding of the specific vehicle.
Tires should be replaced when their remaining tread depth is no longer appropriate for the weather conditions expected to be encountered. Weather conditions vary per location.
Small cuts & punctures in the tread area can usually be repaired by trained personnel using industry approved methods.
Tires can be returned to service if they have not been driven on while flat or with very low inflation pressure.
Driving on a tire while flat or with very low inflation pressure will permanently weaken the tire's internal structure and leave it more susceptible to catastrophic failure.
Tires that exhibit any bubbles, blisters or bulges, or have large cuts, cracks or other significant damage from road hazards in the tread, shoulder and/or sidewall areas should be replaced. Inspecting the tire's innerliner after dismounting the tire from the wheel will reveal the probable cause & help determine if the tire manufacturer's workmanship & materials warranty applies.
Most tires will need to be replaced for other reasons before any prescribed calendar age, therefore the following recommended calendar age removal periods in no way reduces the driver's responsibility to replace worn or damaged tires as needed.
Tires that have been in use for five years or more should be carefully inspected periodically for external signs of aging. While this inspection will confirm the condition of the exterior rubber, it cannot assess internal degradation or damage. Previously unused, never-mounted tires should not be put into service if they are more than six years old even if they were properly stored.