General consensus seems to be at around the 7 year mark for well-maintained tires. Checking and cracking are definite indicators to replace them, but just because they DON'T have checking and cracking, doesn't mean they are in good condition. It will only take one blowout at highway speeds to make you a little paranoid and slightly "rear-end retentive" when it comes to tires. I've had 2 blowouts..... one due to an old tire, (about 10 years old when we first bought our coach), and the other on a brand new tire due to a manufacturer's defect.... although fault was never admitted.... the tire company paid for the damages and replaced the tire.