You are correct. The Pontiac Oakland Club International published an
item in one of their news letters last year on this topic. The rule is
ten years and out for stored tires like the spare, less for all the
The reason why is age hardens the compound so while they look OK, the
compound has lost the ability to flex and grip. UV is speeds up the
process. The item sighted BMW, VW and other manufactors publishing in
the owner's manual warnings on tire age. There is no known test or
inspection to determine how far gone the tires are.
While a bone yard can sell what looks like a good set of tires or a
never used spare, in fact the tire could fail when stressed (ie
high-speed) the first time.
In the US, the issue is with the tire makers no willing to put use by
or born on (like some beers) dates. Fearing the public will reject
tires held in inventory for a year or two over 'fresh'. Congress and
the Feds go around and around on this every few years. The notion is a
year old tire from the warehouse is bad and new is better.
Also mention was limited mileage tires, such as Z rated 20K to 24K
mileage tires. These tires are built to be used up in two to three
years then replaced. Again, the compound is designed to grip the road
and loses that ability over time.
Bottom line is if they are close to ten years old, its time to replace.