The front Continental tyres I have on my Imiev are '08s, and I got them as new old stock for a very low price... £40 for the pair or something. They were wrapped when I bought them shortly after I bought the car which was 4 years old, and I left them wrapped until I needed them after about 6 months. They are fine, no cracks, and hold air perfectly, but I noticed a light layer of oxidation on the side walls that was almost like an application from the factory - it cleaned off with car wash cleaner without much effort, leaving what look like as new tyres. It was sort of a slightly lighter grey/ brown to the normal rubber tyre compound.
There has been talk about limiting tyres to 10 years old max in the UK- following some truck (semi in the US vernacular) incidents - and that is likely a sensible policy for road safety - but will likely take an age to enact into law.
What I have noticed, over the last 20 or so years, is a gradual softening of tyres - prior to the IMIEV I always fitted Michellins - but where I used to get up to 25k miles in the early 90s, I now only get 15k miles or so, from what is essentially the same tyre product range - though the general performance of tyres, especially in the wet has improved hugely.
I would also assume that the compound technology has come a long way, and that age related degradation is less of an issue - but all tyres seem to end up cracking on the side walls first, which tends to track to where the wall meets the tread, as far as I have experienced. I know that road salt isnt good, and the Factory fitted Bridgestones on our Gen 2 Insight had to be replaced at 6 years, one before it reached the end of the tread life, but ive never had that issue with Michellins.
So, I suspect there is no definitive age limit, but rather multiple factors including tyre quality, environment, inflation, driving habit, car weight (and probably suspension characteristics), curb encounters and etc.
As a biker, I would not run any tyre older than 8 years of age, which has come up a few times on some other forums I frequent, but bike tyres are a different story.
I dont think during my entire driving career, I have ever had a flat tyre or any problems during a journey, worst ive had is premature side wall cracking, and of course the odd nail and slow puncture... lucky so far.
Annoyingly with the IMIEV, front/rear tyre rotation isn;t an option, but I still rotate side to side when I have the motivation to do so... I always thought that was good practice.... would have been better for all 4 wheels to be as per the rears I think.... but would likely impede on the already limited front foot-wells and worsen the turning radius.
2012 I-MIEV Keiko Silver 16K
2010 Insight ES-T 43K
2001 Accord Type-V (F23 manual)
2009 Hornet CB600F