Tyre Ageing
All tyres undergo an "ageing process" due to exposure to the environment even if they have never been used or have only been used occasionally
This phenomenon is caused by a number of factors including: the process of aftercuring, whereby the tyre continues to vulcanise very slowly. This is caused by factors such as UV light, heat and movement and makes the tyre become stiffer and more brittle.
oxidation involving oxygen and ozone from the atmosphere causing hardening of the rubber and reduced elasticity
plasticisers drifting from the tread area to other parts of the tyre, resulting in increased hardness.
Tyre ageing can be minimised by proper storage in a cool, dry environment, away from direct sunlight and away from electric motors (such as air conditioning units) or other sources of ozone. Interestingly, tyres, which are infrequently used or not used at all, age and crack quicker than tyres, which are in regular use. For this reason tyre manufacturers tend to recommend including the spare tyre in any tyre rotation process.
How long before tyres become too old?
This is rather like asking "how long can a piece of food be kept before it goes off?" and therefore it is something on which the tyre manufacturers have some difficulty in agreeing. No expert can definitively say how long a tyre will last as a tyre's life depends on many factors such as the type of rubber compounds used, temperature, usage, humidity etc.
However, Bridgestone's website points out that many tyre companies offer manufacturing warranties on tyres for 5 years from the date of manufacture, which has resulted in many vehicle manufacturers advising against the use of tyres that are more than six years old.
In terms of tyres already fitted to vehicles the BRV advises a maximum age of 10 years for car tyres and 6 years for tyres fitted to caravans, trailers of other vehicles likely to be standing around for long periods.
I would not use the original spare tyre and replace it ASAP with a new tyre.