Aged Tyres
There is some evidence to suggest that aged tyres have an increased likelihood of failure due to exposure to the environment. Ideally tyres should be kept in a cool, dry environment out of direct sunlight and away from electric motors or other sources of ozone. If these conditions are not met a tyre may have an increased chance of failure. It is for this reason that Bridgestone recommend including full size spare tyres in the rotation schedule.

How do I tell the age of a tire?
All tyres are produced with a serial Tyre Identification Number (or serial TIN) that shows the date of manufacture of a tyre (See Figure 1&2 below). The last three digits (for tyres made pre 2000) or four digits (for post 2000 tyres) of the serial TIN indicate the week and year that the tyre was made. For example, Figure 1 below shows a tyre made in the 37th week of 1995 and Figure 2 shows a tyre made in the 12th week of 2004. Also a tyre made in the 1990's can be distinguished from a tyre made in the 1980's due to a triangular indentation after the last number which is not present on 1980's tyres.


Figure 1: Serial code for 37th week of 1995

Figure 2: Serial code for 12th week of 2004

How old is too old?
This is a subject of much debate within the tyre industry and no tyre expert can tell exactly how long a tyre will last. However, on the results of experience many tyre companies, including Bridgestone, warrant their tyres against manufacturing and material defects for five years from the date of manufacture. Based on their understanding a number of vehicle manufacturers are now advising against the use of tyres that are more than six years old due to the effects of ageing.

Tyre Ageing Mechanism
There are three main mechanisms of tyre ageing. The first involves rubber becoming more brittle. Sulphur is used to link rubber molecules together during vulcanisation with the application of heat and pressure, giving the rubber its useful elastic properties and strength. As the tyre absorbs energy in the form of light, heat or movement the tyre continues to vulcanise. This ongoing vulcanisation causes the rubber to become stiffer and more brittle.
The second mechanism of tyre ageing is oxidation involving oxygen and ozone from the air compromising the strength and elasticity of the rubber and the integrity of the rubber to steel bond. Basically heat and oxygen cause cross linking between polymer chains (causing the rubber to harden) and scission of polymer chains (leading to reduced elasticity).
Thirdly, breakdown of the rubber to steel-belt bond will occur due to water permeating through a tyre and bonding with the brass plate coating on steel belts. This causes the steel to rubber bond to weaken leading to reduced tyre strength and reduced heat resistance. If compressed air used for inflation is not completely dry, tyre strength will be affected over time. Even unused tyres will become more brittle, weaker and less elastic with exposure to water, air, heat and sunlight.

Warning signs
Regardless of their age tyres should be replaced if they show significant crazing or cracking in the tread grooves or sidewall (Figure 3&4) and or bulging of the tread face or sidewall. All tyres, especially unused spare tyres, should be inspected periodically to determine their suitability for service. If there is any question about a tyre's suitability please consult your local Tyre Centre expert.

Figure 3: Sidewall cracking due to environmental exposure to oxygen, heat and sunlight.

Figure 4: Tread cracking due to environmental exposure to oxygen, heat and sunlight.


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Source:
http://www.bridgestone.com.au/tyres/passenger/care/age.aspx
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