What age to replace tires?

Tires are built to provide thousands of mi./km of excellent service. For maximum benefit, tires must be maintained properly to avoid tire damage that may result in removal from service before the tread is worn down to minimum depth. It is not practical to accurately predict the service life of any specific tire in chronological time since service conditions vary widely. The serviceability of a tire over time is a function of the storage and service conditions (inflation pressure, load, speed, road hazard injury, etc.) to which a tire is subjected. Consumers should not rely solely on the appearance of the tire but should be aware of any change in dynamic performance such as increased air loss, noise or vibration, which could be a sign to remove the tire. Therefore, it is essential to have tires, including spares, inspected regularly (at least monthly) for proper inflation pressure, damage and treadwear.