Just came back from the Rally (in Syracuse) and it seems to be the consensus that you replace ties at 5-6 years. My tires were made on 9/07 so I am ready to bite the bullet. Got a quote from a local, ...
The NHTSA states that rubber degradation increases exponentially after 5 years primarily from ozone. Tires should be inspected regularly regardless of age and have more frequent inspection at 5 years. Tires should be replaced at the first signs of problems developing.