As for the structural integrity of the tire, age is just as important as it is for some brands of beer or liquor. Only thing is that older liquor is better and older tires are not. Tread showing on the tire is of no importance in this situation, but would be in other situations.
One of our service writers was asked what kind of tire to buy. He immediately replied "Michelins". He went on to talk about their great quality and other attributes. In many instances, his comments would be correct, but in a dealership or repair shop situation, such statements can come back to haunt.
For example, if an elderly lady that "wants the best tire I can buy" gets a set of 80,000 mile tires and puts on her car, that's fine. It stays garaged and she drives gently--only once or twice a week at best. Hence, few miles per year compared to a normal driver that could well do 15,000 miles/year.
Five years later, the 15,000 mile/year driver is needing new tires. Tread's not quite passing state inspection so it's time for tires. The elderly lady's tires still have ample tread left and have been well cared for with no flats or any other problems. Things still look good so she has "good tires".
In a few more years, her service provider notes that the sidewalls and tread areas are getting weather/age cracked. She recalls that those tires were supposed to last 80,000 miles and she's only got about 20,000 miles on them. In other words, she's taken those earlier recommendations to heart and that's the way it will be. A few months later, she goes out and finds that one is flat and not repairable due to separation. The consensus is that if that one's that way, the others can't be far behind. Seeing that evidence, she reluctantly buys new tires. Next time she sees that young man that thought so highly of Michelins, she'll probably verbally whop him over the head with her purse!
In the times of bias ply tires, we never saw these issues as most of them were worn out inside of 40,000 miles. Many looked like they'd go farther, by watching the tread wear rates versus mileage, but when they got to that last 1/4 of tread depth, it seemed that wear rates were accelerated somewhat.
As someone recently noted with trailer tires, it's the non-use or idle times that do more damage to the tire than constant use. In that respect, an 80,000 miles tire that is used daily could well last to 100,000 miles but with very little use might be doing good to last 30,000 miles.
Over the years, I've heard tire people talk about moisture and dry rot of the cords. Although the carcass of the tire is sealed, it seems that moisture can still creep into the tire with hot/cold cycles. If the tire is used regularly, the heat of operation probably keeps things "cleaned out", but with non-use, they get in there and stay. Kind of like an engine that sees short trips and one that is used for longer trips each time it's started--except that you can't change the oil in the tire carcass and get all of the moisture-laden oil out of it.
To me, the key thing is to buy tires to last about 3 or 4 years for the miles you drive each year. That might get you out of the "high priced" high mileage rated tires, but it would also mean that tire aging issues should not be a problem. It would also require you to buy new tires sooner as if there's still lots of tread on the existing tires, there'll always be a reason not to buy tires as the ones you have are still good (or so it seems). These are judgment calls that each respective driver must make themselves.
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