Many of the antique car forums discussed this a while back. It was stated that BFG will not warranty any tire more than six years old, even if it had been in their warehouse all of that time. Many posters related incidents with tires that had plenty of tread left on them, but separated while driving to car events (cross country). They might hold air, might have plenty of tread left, and they might last decently when on the ground, but if you have to make a sudden lane change or similar, it could cause things to separate internally.
In prior times, there was a tire rotation scenario that included the spare tire, but few people used it back then. At best, you should consider it a "limited use" situation even if it's still got all of the casting flashes on it and whatever coating it might have come with. From what I've gathered from talking to many tire people about "stored tires", they can develope "dry rot" internally and look great on the outside.
As mentioned, tires on outside tire carriers on pickup trucks or SUVs might need replacement sooner than one hidden in the trunk of a car. Might be a situation where it's the owner's judgment call, but the warranty comments from BFG can be useful too.
Just some thougths,
CBODY67