Jon,

The only point of my post was to illustrate there is no relationship between a warranty period and tire replacement criteria (which is a good thing, Tom). The Michelin Service Life Booklet and Bridgestone's How Old is Too Old Brochure are not warranty documents but are published to assist the tire user is determining when to change a tire. Michelin's only reference to replacing a tire based on age appears in their Service Life Booklet:

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.
A simular quote from Bridgestone's "How Old is Too Old":

RV users, for example, often put no more than a few thousand miles on their tires a year. Their tires may need to be replaced because of age long before their treads are “worn out.” Is a 10-year-old tire too old? Probably. Is a 6-year-old tire too old? Maybe. Have your tires inspected. The age of your tires, along with their overall wear, condition of sidewalls, etc. are all factors your tire dealer will take into consideration when inspecting your tires and advising you on tire replacement.
I agree it is not realistic to compare our bus tires to other applications. BUT, as in ALL tire applications, tire condition, ride performance and environmental exposure are major considerations for deciding whether or not to replace a tire.

As always MOST of us will defer to your conservative and thoughtful advic. You will remember however that you have occasionally admitted that if you do not have the answer for a question you will make one up. In this case it appears you may be trying to justify your personal tire change criteria. I for one, would replace my tires at 5 years in an instant if you could offer evidence that age should be the determining factor in making that decision.
Incidentally, for a guy that is so darn careful about tire condition, it amazes me that you would expose the lovely Di to the lurking unknowns of Chinese tyres