How old is too old is an oft-asked question. While the industry guidelines say six years in use or 10 years from build, your experience can vary. The life span you see from any tire can vary greatly depending on how it is used. For example, counter to common belief, an infrequently used tire actually lives a harder life than one that is exercised on a regular basis. Since most people relate tire life to number of miles driven, this can come as a surprise. However, rubber needs to be exercised to remain in top condition. Sitting around for extended periods of time, then being asked to go into full service can result in flatspotting (both temporary and permanent), as well as cracking. This is more likely to happen when the tires remain mounted on a vehicle and sitting idle with the vehicle weight on them, as opposed to stored off the vehicle.
Once you go past either the six years in use or 10 years from build time frame, it is recommended to have the tires inspected by a professional to insure they are still safe to be used. It would be best to have this inspection done at least once a year.
More information on what the industry says about tire aging is available here? Keep in mind that the in-use time starts as soon as the tire is exposed to the elements. For example, your under-the-truck-bed mounted spare is considered in use. Do you have a spare sitting on the back of your Jeep? That's considered in use, too. You can check the production date of your tire very easily to see how old it is. Read "How Old Are the Tires I Buy?" for more details.
Have your tires been in use for longer than they should be? Are you ready for a new set? Shop by vehicle and find a great replacement option for your car.