I have often wondered about this, so did some research (though of course, research on the internet is hardly the most scientific source):
Michelin say:
If the tyres have not been replaced 10 years after their date of manufacture, Michelin recommends replacing them with new tyres as a precaution. This recommendation also applies to spare tyres.
https://www.michelin.co.uk/auto/advice/change-tyres/how-long-do-tyres-last
Goodyear say:
While most tires will be replaced sooner, Goodyear recommends that any tire in service (meaning inflated and mounted on a rim of your vehicle, including your spare tire regardless of whether that tire is in contact with the roadway) 6 years or more be replaced even if such tire appears serviceable and even if it has not reached the legal treadwear limit.
https://www.goodyear.com/en_US/learn/tire-basics/how-long-do-tires-last.html#:~:text=While%20most%20tires%20will%20be,and%20even%20if%20it%20has
Vredstein sit on the fence:
Car tyres are composed of various rubber components, each of which is subject to a certain amount of ageing. The extent to which a tyre ages depends on various factors, such as the number of kilometres driven per year, the frequency and duration of use (e.g. daily or a few times per year), the speed driven and the regularity of tyre pressure checks. Weather conditions, tyre load (i.e. normal load or maximum load) and the way in which the tyre is stored when not in use are all factors that affect the ageing process.
As a result of all of these different factors, it is impossible to say exactly how many years or how many kilometres a car tyre can be used. Remember that the older the tyre is, the greater the chance that it will need to be replaced.
For a tyre to have as long a lifespan as possible, we recommend that you have it occasionally checked by a tyre specialist in addition to carrying out a monthly check yourself. If the minimum tread depth of the tyre has not yet been reached after six years, we advise having the tyre checked by a tyre specialist at least once a year.
Please note: Ensure that extra attention is paid to tyres fitted on vehicles such as trailers, caravans, camper vans, boat trailers and horse boxes. As tyres in these applications are only used from time to time and are constantly subjected to maximum loads during use, they can age more quickly.
https://www.vredestein.co.uk/content-hub/tyre-guide/car-suv-van/when-should-i-replace-my-tyres/
Vintage tyres say:
While tyres made with modern construction methods are pretty good at standing the test of time, they are not invincible and will still start to deteriorate after the first use. Vintage Tyres recommends that you replace your tyres every 10 years.
Even if the tread depth still looks okay, after 10 years, the tyre will have vulcanised/hardened to such a point that it would be quite dangerous to drive on, particularly in damp conditions.
https://www.vintagetyres.com/blog/how-old-are-your-tyres.htmlOf course, all of these sources have a vested interest in selling you tyres, so you could argue ‘They would say that, wouldn’t they’. But on the other hand, they do a decent level of expertise with their products.
On the other hand, this scientific study on the effect of aging, is manufacturer independent, and suggests that 'The study implied that age did have effects on safety performance of the tyres and significant difference in terms of safety performance was observed between 'Marked' and 'Non-marked' tyres.' and that quality tyres should be replaced every 7 years.
https://jsaem.my/journal/article/view/141
