Tyre life

The following is an extract from the B F Goodrich web site and seems to give a bit better life expectancy than a lot of seemingly unqualified opinions around the traps.

It is impossible to predict when tires should be replaced based on their calendar age alone . However, the older a tire the greater the chance that it will need to be replaced due to the service-related evolution or other conditions found upon inspection or detected during use. While most tires will need replacement before they achieve 10 years, it is recommended that any tires in service 10 years or more from the date of manufacture, including spare tires, be replaced with new tires as a simple precaution even if such tires appear serviceable and even if they have not reached the legal wear limit. For tires that were on an original equipment vehicle (i.e., acquired by the consumer on a new vehicle), follow the vehicle manufacturer's tire replacement recommendations, when specified (but not to exceed 10 years).
The date when a tire was manufactured is located on the sidewall of each tire. Consumers should locate the Department of Transportation or DOT code on the tire which begins with DOT and ends with the week and year of manufacture. For example, a DOT code ending with "2204" indicates a tire made in the 22nd week (May) of 2004.