For years, people have relied on tread depth to determine when to replace a tire. If the tread passes the "penny test," they assume the tire still has life, regardless of how old it is, which can be a fatal mistake. Old tires are dangerous, regardless of tread depth. While there's no federally sanctioned safety guidance on when a tire is too old to be safe, many carmakers recommend replacement at six years from the date of manufacture.

Old tires have been the culprit in fatal accidents. Here are just two examples:

In 2008, the owner of a 1998 Ford Explorer in Georgia needed a new tire for his SUV and bought a used one. When he was driving two weeks later, the tread suddenly separated from the tire. The Explorer went out of control and hit a motorcycle, killing its rider. An analysis of the used tire revealed that it was nearly 10 years old.

In a more recent and high-profile example, the investigation into the cause of the 2013 accident that killed the actor Paul Walker revealed that the Porsche Carrera GT in which he was riding had 9-year-old tires. The California Highway Patrol noted that the tires' age might have compromised their drivability and handling characteristics, according to the Los Angeles Times.

These incidents illustrate the potential danger of buying used tires and the perils of driving on aging tires — including those that have never spent a day on the road. The rubber compounds in a tire deteriorate with time, regardless of the condition of the tread.

For some people, old tires might never be an issue. If you drive a typical number of miles, somewhere around 12,000-15,000 miles annually, a tire's tread will wear out in three to four years, long before the rubber compound does. But if you drive much less than that, or have a car that you only drive on weekends, aging tires could be an issue.

Similarly, if you are buying a used car, there's a chance it may be riding on old tires. The age warning also applies to spare tires and seemingly new tires that have never been used but were produced years ago.

What Happens to a Tire as It Ages?

Sean Kane, president of Safety Research & Strategies Inc., compares an aging tire to an old rubber band. "If you take a rubber band that's been sitting around a long time and stretch it, you will start to see cracks in the rubber," said Kane, whose organization is involved in research, analysis and advocacy on safety matters for the public and clients including attorneys, engineering firms, supplier companies, media and government.

Cracks in a tire's rubber begin to develop over time, appearing on the surface and inside the tire as well. This cracking can eventually cause the steel belts in the tread to separate from the rest of the tire. Improper maintenance and heat accelerate the process.

Kane and his organization have identified over 250 incidents in which tires older than six years have experienced tread and belt separations — most resulting in loss-of-control and rollover crashes. These incidents were the cause of 233 fatalities and 300 injuries in 2012.

Every tire that's on the road long enough will succumb to age. Tires that are rated for higher mileage have antiozonant chemical compounds built into the rubber that will slow the aging process, but nothing stops the effects of time on rubber.

How Long Does a Tire Last?

Carmakers, tiremakers and rubber manufacturers differ in their opinions about the lifespan of a tire. The National Highway Traffic Safety Administration (NHTSA) has no specific guidelines on tire aging and defers to the recommendations of carmakers and tire manufacturers.

Many automakers, including Ford, Nissan and Mercedes-Benz, tell owners to replace tires six years after their production date, regardless of tread life. Tire manufacturers such as Continental and Michelin say a tire can last up to 10 years provided you get annual tire inspections after the fifth year.

The Rubber Manufacturers Association said there is no way to put a date on when a tire "expires" because such factors as heat, storage, underinflation and conditions of use can dramatically reduce the life of a tire. Here's more on each of these factors:

Heat: NHTSA research has found that tires age more quickly in warmer climates. NHTSA also found that environmental conditions, such as exposure to sunlight and coastal climates, can hasten the aging process. People who live in coastal states and other areas with warm weather should keep this in mind when deciding whether they should retire a tire.

Storage: This applies to spare tires and tires that are sitting in a garage or shop. A tire that has not been mounted and is just sitting in a tire shop or your garage will age more slowly than one that has been put into service on a car. But it ages nonetheless.

Spares: They usually don't see the light of day, but they're still degrading with time. If the tire has been inflated and mounted on a wheel, it is considered to be "in service," even if it's never been used. And if a truck's spare is mounted underneath the vehicle, it's exposed to heat, dirt and weather — all reasons to plan on replacement.

Conditions of use: This refers to how the tire is treated. Is it properly inflated? Underinflation causes more tire wear. Has it hit the curb too many times? Has it ever been repaired for a puncture? Tires on a car that's only driven on the weekends will age differently from those on a car that's driven daily on the highway. All these factors contribute to how quickly or slowly a tire wears out.

Proper maintenance is the best thing a person can do to ensure a long tire life. It is important to maintain proper air pressure in tires, rotate them regularly and get routine inspections.

How to Determine the Age of a Tire

The sidewall of a tire is covered in numbers and letters. They all mean something, but deciphering them can be a challenge. This Edmunds article about reading a tire's sidewall goes into greater detail. But to determine the age of a tire, you simply need to know its U.S. Department of Transportation (DOT) number.

Tires made after 2000 have a four-digit DOT code. The first two numbers represent the week in which the tire was made. The second two represent the year. A tire with a DOT code of 1116 was made in the 11th week of 2016.

Tires made before 2000 have a three-digit code that is trickier to decode. The first two digits still indicate the week, but the third digit tells you the year in the decade that the tire was created. The hard part is knowing what decade that was. Some tires made in the 1990s (but not all) have a triangle after the DOT code, denoting that decade. But for tires without that, a code of "328" could be from the 32nd week of 1988 — or 1978. Really, you can ignore all that: If you see a DOT number ending in three digits, the tire was made in the last century and needs to be replaced as soon as possible.

Clearly, these DOT numbers weren't designed with everyday buyers in mind. They were originally put on tires to make it easier for NHTSA to recall tires and keep track of their manufacturing date.

To make matters worse, you might not always find the full DOT number on the outer side of the tire. Because of the way a tire is made, it is actually safer for the technician operating the mold to imprint information on the inner side of the tire, so some manufacturers will opt to put the number there. It is still possible to check the DOT code, but you might have to jack the car up to see it. Keep the visibility of the DOT number in mind the next time you are at a tire shop and the installer asks if you want the tires to be mounted with the raised lettering facing in.

After checking out a tire's birthdate, give the rubber a visual inspection. Some of the best advice on such an inspection comes from the British Tyre Manufacturers' Association. It recommends that consumers check tires regularly for any sign of aging, such as tread distortion or large or small hairline cracks in the sidewall. Vibrations or a change in the dynamic properties of the tire could also be an indicator of aging problems, the association said. It recommends replacing the tire immediately if such symptoms appear.

Don't Buy Used

Tires are expensive, especially when you factor in the price of mounting and balancing. That's why used tires become more attractive to people who are strapped for cash, and so some small and major shops continue to offer used tires as an alternative. But when you buy a used tire, you have no idea how well it was maintained or the conditions in which it was used. The previous owner might have driven it with low pressure. It could have hit curbs repeatedly. It could have been patched for a nail. You can check its age, but it's better to avoid it entirely.

Make Sure You're Getting a "Fresh" Tire

Just because a tire is unused doesn't mean it's new. In a number of instances, people have purchased "new" tires at retail stores only to find out that they were manufactured years earlier. In addition to having a shorter life on the road, a tire that's supposedly new but is actually old may be past its warranty period.

If you buy tires and soon after discover that they're really a few years old, you have the right to request newer ones. Any reputable store should be willing to make amends. But to save yourself the hassle, check the date before you buy.

Letting Go

Getting rid of an unused spare or a tire with plenty of tread may be the hardest thing for a thrifty owner to do. "Nobody's going to take a tire that looks like it's never been used and throw it out," Kane said. But if it's old, that's exactly what the owner should do.

Although Kane has lobbied NHTSA to enact regulations on tire aging, nothing is currently on the books. A NHTSA spokesman said the organization is "continuing to conduct research into the effects of tire aging" and what people can do to monitor their tires for safety.

Since there's no consensus from government or industry sources, we'll just say that if your tire has plenty of tread left but is nearing the five-year mark, it's time to get it inspected for signs of aging.

Of all your vehicle's components, tires have the greatest effect on the way it handles and brakes. So if the tire store recommends new tires at your five-year checkup, spend the money and don't put it off. Your life could depend on it.