I try to check for dry rot as well as wear. Don’t go by a specific age. I do have an old tire story. I saved the original white wall tires off my first car, a 1981 Buick Regal with tires that were probably about circa 1988. Those tires eventually found their way on to the boat trailer (18′ bass boat). In about 2000 or so I was towing the boat and the tread completely separated from the tire in one giant chunk. These were NOT retreads, just 12-15 year old tires.

I’m a cheap guy, but I don’t fool around with tires and brakes. Since I’ve had lots of beaters, I’ve bought lots of tires, too…both based on tread and age. 3-4mm is the minimum acceptable tread, and 6-7 years the age barrier. We have a pretty mild, wet climate and little harsh sun, so I figure it is winters with salted roads that cause the most tire wear.

I didn’t change based on age until I bought the Miata. From what I’ve heard from fellow owners, to really have fun with the car safely you really need new tires every four years or so. The tires that were on it when I bought the car were 7 years old and were kind of scary to drive on in the rain. In second gear, going through a left turn, the car would go sideways. I could catch it, but really I’d prefer it do that in a more gradual and predictable way, especially if they’re only having to control 128hp. Then I bought some used earlier wheels, which were lighter, but the tires were a year older. So now I’m on new Sumitomos and am happy with them. In the long run, I’d like to change up to 205/50R15, for performance and because I’ve just always loved the look of the proportions of the 195/50 and 205/50R15. With that change, I’d like to go with more aggressive rubber, which definitely will mean regular changing by date. On the daily driven Volkswagen, age isn’t really an issue. It’s driven enough for the rubber to remain soft.

When I bought my wheels & tires for the T’bird at Discount Tire, they tried to sell me their lifetime free replacement certificates. I declined, citing the low mileage the tires would see. He replied that’s why i should buy them, they aren’t allowed to service (balance, repair, etc) tires older than 6 years, I think. If I came in with good tread and old tires, the replacement certificates would get me a new set, free. I bought the certificates.
It’s been 7-8 years and I intend to take it in this summer for a balance to see if they make good on it.

I have done so in the past but am not completely diligent about it. I don’t put many miles on any one vehicle, so they don’t generally wear out. I will replace tires when I see sidewall checking or any hint of tread separation.

I recently had a small pickup as a third vehicle. I maybe drove it 3000 miles a year. The no-name Chinese tires that the previous owner had on it started to look questionable (ample tread but sidewalls started to look wonky). I replaced them with halfway decent new major brand tires this last year for peace of mind.

I’m running the same tires that have been on since before 1998? A little weather checked, me no problems yet?

The U.S. National Highway Traffic Safety Administration (NHTSA) and
tire manufacturers are currently studying the many variables. Exposure
to the elements (sun and atmospheric), regularity of use (frequent or
only occasional) and the quality of care (maintaining proper inflation
pressure, wheel alignment, etc.) will all influence the answer. So while
tire life depends on the service conditions and the environment in
which they operate, the difficult task remains how to identify all of
the variables that influence a tire’s calendar age and attempt to
quantify their influence.
The current industry association recommendations regarding inspecting and
replacing tires due to age originate outside the United States.
The British Rubber Manufacturers Association (BRMA) recommended practice
issued June, 2001, states “BRMA members strongly recommend that unused
tyres should not be put into service if they are over six years old and
that all tyres should be replaced ten years from the date of their
manufacture.”
“Environmental conditions like exposure to sunlight and coastal climates, as well as
poor storage and infrequent use, accelerate the aging process. In ideal
conditions, a tyre may have a life expectancy that exceeds ten years
from its date of manufacture. However, such conditions are rare. Aging
may not exhibit any external indications and, since there is no
non-destructive test to assess the serviceability of a tyre, even an
inspection carried out by a tyre expert may not reveal the extent of any
deterioration.”
More recently, The Japan Automobile Tire Manufacturers Association (JATMA)
recommended practice issued May, 2005, states “customers are encouraged
to have their vehicle tires promptly inspected after five years of use
to determine if the tires can continue to be used (recommends spare
tires be inspected as well). Furthermore, even when the tires look
usable, it is recommended that all tires (including spare tires) that
were made more than ten years ago be replaced with new tires.
Additionally, because in some cases automobile makers–based on the
characteristics of the relevant vehicle–stipulate in the owner’s manual
the timing of tire inspection and replacement. Please read and confirm
the content of the owner’s manual.”
Several European vehicle manufacturers of high performance sports cars, coupes
and sedans identify that “under no circumstances should tires older than
6 years be used” in their vehicle owner’s manual. However, it should be
noted that European recommendations must include driving conditions
that include roads like the German Autobahn, which allows vehicles to be
legally driven at their top speeds for extended periods of time.
While American driving conditions don’t include the high-speed challenges of
the German Autobahn, Chrysler, Ford Motor Company and General Motors
have joined their European colleagues by recommending that tires
installed as Original Equipment be replaced after six years of service.
It is important to take into account Original Equipment tires are mounted
on wheels and put into service right after being received by vehicle
manufacturers, so their calendar age begins immediately. However the
same cannot be said of tires properly stored in a tire manufacturers’
warehouse or in Tire Rack distribution centers before they go into
service. Properly stored tires that are protected from the elements and
not mounted on a wheel age very slowly before they are mounted and put
into service.
Our experience has been that when properly stored and cared for, most street tires have
a useful life in service of between six to ten years. And while part of
that time is spent as the tire travels from the manufacturing plant to
the manufacturer’s distribution center, to the retailer and to you, the
remainder is the time it spends on your vehicle.

Generally speaking you’ll hit the usage threshold (4mm profile by law in Germany) far earlier than the age threshold (10 years advisable), at least I always have. Tires however aren’t always sold immediately after production and can lie in storage for a number of years before going over the counter. It’s advisable to check that new tires you buy aren’t older than three years since production (there’s a DOT number on the sidewall, 2311 for example stands for the 23rd week of 2011).
I am bound by law about trailer tires though, being younger than six years is one of the binding requirements if you want to tow at 100 kph / 62 mph, otherwise you’re limited to 80 kph / 50 mph.

Good topic! Tires tend to be overlooked…along with brakes and front-end parts, y’know, the stuff nobody really cares about until it fails.
If you’re now curious to see how old your tires are, it’s easy to figure out:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=11
Date codes are a huge deal for certain types of vehicles, primarily RVs and motorcycles, due to the (typical) low annual mileage racked up on them.
This said, I’ve purchased three tires for a motorcycle at one time, went to the shop, gave ’em all three, and they asked where the other rear wheel was (the second one was a rear).
This was on a Wednesday, they’d be ready Thursday afternoon. Perfect.
I said, “oh, just keep that one over on the side, somewhere, I’ll need it on Monday.”
They looked at me with a hairy eyeball, and sure enough, I was back in on Monday with a shot rear off my ’91 ST1100.
Iron-Butt style riding does that to a rear tire, especially when you’re two-up and your wife/GF/pillion trusts you to the very limits of physics.