Michelin goes out on a limb and says that a tire up to 10 years old - in use - is fine as long as you give it an annual inspection every year after year 5. A tire in storage - not in a hot trunk or underneath a car as a spare - but rather in a building or garage will age much more slowly than one in use that is exposed to the elements. Really, not that much is known about aging tires in benign environments.
I have had 12 year old Michelins with over 85,000 miles on my F-150 the rubber was hardened but they still ran fine. However I won't install a tire over 18 months old on any of my bikes. If you have an issue with 4 tires on a cage YOU may get to pick up pieces of TIRE off the road. If you have an issue with 2 tires on a bike THEY may get to pick up pieces of YOU off the road. Just not worth it in my judgement..