More great advice from the poster who ran their engine dry and wants to fix it by taking photo's.

The advice from all sensible sources is that tyres over 6 years old are past their best and should not be used. Tyres that are used infrequently will age faster and show cracking much sooner.

If there is cracking scrap the tyre, they are your only contact with the road and if they fail at speed you have an accident, you ,may die, you may kill others, if you live and your tyres are found to be faulty you will be prosecuted."

This is not advice, nor do I represent it as such. Its a (tentative) idea which I'm experimenting with.

Re tyre lifespan of six years, (mine are about 8) depends what you consider a "sensible source." Ford and Chrysler both recommend 6 years. They get sued a lot, as you might expect. This gives them a vested interest slightly different from tyre manufacturers, who don't want to get stuck with ageing inventory.

The most specific statements from the tyre industry suggest 10 years, but latterly they've tended to defer to the car manufacturers recommendation, which are often lower. This deference is due largely to damaging US litigation between Bridgestone, who produced some sub-standard tyres, and Ford, who recommended a very low tyre pressure for the Explorer, which tended to roll-over and kill people when they burst.

Does this make Ford a "sensible source"?

This episode, and subsequent investigation, produced most of the publicly available information on tyre failure mechanisms.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=138

The article quotes the two tyre industry associations that have made recommendations.

“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."

The British Tyre Manufacturers Association more recent recommendations, in contrast, say b*****-all, referring the reader to the vehicle owners handbook.

http://www.btmauk.com/data/files/Tyre_service_life_recommendations_31_May_2011.pfd
Mine is in Chinese. I'd bet the price of a new set of tyres that it says b*****-all too, but then its 25 years old.

Perhaps more relevantly for local conditions for me here in Taiwan, they also refer to The Japan Automobile Tire Manufacturers Association (JATMA) who's recommended practice issued May, 2005, is quoted as stating:-

" it is recommended that all tires (including spare tires) that were made more than ten years ago be replaced with new tires." though they also attempt to pass the buck to the owners handbook.

So both these recommendations, including the one from the famously anal Japanese, [EDIT: That should perhaps now be "famously (at least until Fukushima) anal Japanese"] are for 10 years.

These are of course general recommendations and have to cater for some very high performance vehicles and jurisdictions where there are no speed limits. Those conditions do not apply to my sub-litre car, especially when driven here, by me.

Bridgestone-Firestone apparently adopted the Japanese recommendation, with the caveat that there is/was no specific technical basis for it.

http://www.safetyresearch.net/2006/01/01/number-of-tire-age/
“we believe it is appropriate to follow the JATMA recommendation in the interest of further encouraging consumers to focus on the importance of maintaining and properly replacing their tires.”

They also defer to the owners handbook.

http://thesafetyrecord.safetyresearch.net/2010/11/18/tire-age-issue-still-languishing-in-us/

The OTHER judgement-call is "How bad is too bad" re the sidewall cracking, and that's controversial and subjective too. The only actual standards I've seen, are that (a) you shouldn't be able to see the internal belting. (e.g. UK MOT) and that (b) cracks shouldn't be more than 2/32" (USA, sorry. That's about 1.6 mm in the developed world) but I dunno how the crack depth is operationally defined/measured.

Beyond that its subjective, but I've run lots of tyres in the UK with fine surface cracks that I consider cosmetic. This ref matches my predjudices nicely. I've got lots more, but I'll spare you.

http://fleetowner.com/management/exercise_tires_0809

"The weather-checking or “dry-rot,” as it's known, poses no risk to the performance of the casing in its minor stages. In fact, in order for it to result in any damage, it must extend to the steel body cables or be greater than 2/32 of an inch deep. Of course, not all sidewall cracks should be ignored. Large circumferential cracks about an inch above the rim flange and small cracks near the shoulder should be inspected by a tire professional. But just like the theory that all roadside tire debris is the result of faulty retreads, the court of public opinion has already determined that all tires with sidewall cracks should be immediately removed from service."

So I stand condemned in the court of public opinion? Oh dearie me.

Since the UK MOT simply requires that the belting not be visible, (which is rather more liberal than I would choose to be) it seems unlikely that fine sidewall cracks could be a basis for a prosecution in the UK, as you state above. (I'm assuming the UK is your legal reference frame).

In the US one could probably be sued for this, as for just about anything else.

My reading suggests that, while UV is a factor, the main threats are oxygen and ozone. Intuitively, UV would not be expected to penetrate deep into the structure of the tyre, but oxygen from the pressurised air content does permeate it.

Surface damage from ozone and UV is self-limited by a thin layer of degraded rubber and carbon black, but in a crack, fresh rubber under tension is continually exposed, and "snipped" by ozone attack. This mechanism means cracks can propagate relatively rapidly.

My model is that the sunflower oil fills the cracks so the fresh rubber is not exposed to ozone. Since sunflower oil is a relatively reactive, "semi-drying" oil containing a lot of double bonds, it should also react preferentially with the ozone.

The sidewalls of tyres are attacked by UV light from the sun. In the UK it is generally accepted that tyres that have been in use for 10 years should be removed. The internal damage is invisible.

I think the sun/UV effect is the most significant, plus the need to run at the correct pressure to limit tyrewall flexing. A recommended lifespan for a tyre is unrealistic because its use and exposure to UV can vary so widely. And since cracking is accelerated by UV I am fairly sure that the interior will be in better condition, especially as it will be under compression rather than tension.

I have 1990 Michelins on my 1990 Peugeot which I keep in a garage and drive every couple of weeks. The sidewalls show virtually no cracking, and when the annual MoT tester advises me to change them for that reason I shall do so.