When to Replace Your Tires

Replace Old Tires Even if There is Tread Remaining Vehicle Manufacturers Recommend Replacement at 6 Years Tire Manufacturers' Warranties Expire at 6 Years Tire Manufacturers Recommend Replacement at 10 Years Industry Experts Recommend Replacement at 10 Years
Certain chemicals added to new tires allow rubber to be soft and flexible. Over time and as air migrates through the tire, the chemical's effectiveness weakens, allowing rubber to become more brittle and lose strength.
Consumer Advisory: Factors to Consider in the Life of Your Tires
The following elements each play an important part in your tire's safety. Throughout the life of the tires it is necessary to see how each of these plays a different role. Excludes trailer tires. Up to 6 Years: Visual tire inspections and monthly air pressure checks are recommended. Tire Quality/Construction: Features and Benefits help to describe capabilities of tires during this period. Service Conditions/Maintenance: Rotate tires every 6-8,000 miles, check air pressure monthly and check tire balance every 12-16,000 miles. Tire Manufacturers suggest most tires are out of service at 3-4 years based on wear. Tire Wear/Condition: Less than new tread changes traction and stability capabilities in extreme weather conditions (such as: snow, ice, rain, dirt/mud). Environmental Conditions: Exposure to heat and ultraviolet rays may cause structural changes in the tire not found in more moderate climates. Tire DOT Number*: Tire age is not the major consideration during this portion of the tire's life.
6 to 10 Years: Replacement is recommended. Tire Quality/Construction: Are more valid concerns as some tires are designed to be nearing the end of their service life based on average consumer travel of 12-15,000 miles annually. Service Conditions/Maintenance: Tires that have not been serviced or maintained properly are typically at the end of their service life. Tire Wear/Condition: Less tread reduces traction and stability in all weather conditions as well as propensity to punctures. Environmental Conditions: Exposure to heat and ultraviolet rays causes ozone/weather cracking and structural changes. Tire DOT Number*: Now, one of the important considerations as some vehicle manufacturers recommend replacement and tire manufacturer warranties expire.
More Than 10 Years: No service on tires with a DOT beyond 10 years. Tire Quality/Construction, Service Conditions/ Maintenance, Tire Wear/Condition, Environmental Conditions: Regardless of all of these conditions, tires reach the end of their life. Tire DOT Number*: Tire age is the most important consideration during this portion of the tire life as tire manufacturers recommend replacement of any tires regardless of service, including spares. * Department of Transportation Number is stamped on the sidewall of every tire. The last group of digits indicates the week and year the tire was built.
(source :  http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireLife.jsp)