My six year old TT has 7,000 running miles on it and other then while we are camping (about 30 days a year) don't see the sun. After six years they look like new tires (very little wear and no sidewall cracking).
Unless there are alignment issues that will show up via an unusual wear pattern, a lot of miles, long exposure to sunlight or running without proper inflation I don't think a fixed time period can be set for when tires should be replace. Tread wear, side wall condition and years of service are connect but the critical factors for me are wear and side wall condition rather than age alone as the prime indicators of when tires need to be replaced.