Longer lifetime of a Michelin tyre is not achieved by sacrificing the grip or by rapid aging of the rubber.
The characteristics of the tyre change during the use both because of wear and because of change in rubber. That just has to bee taken into account when driving. It is a strong exaggeration to say, that the tyres become immediately dangerous when getting older and harder. They just change. But withing first 15 years that change is easily less significant than the difference between the different brand new tyres, provided that here is enough tread.
Personally I drive the tyres as long as they are legal and performing sufficiently. Typically this means 3-4 winters for winter tyres and 4-8 summers with summer tyres. But I have also used with driven 20+ year old tyres, which has been in minimal use in the beginning and gone to me as used with some older car. If there is enough tread, they are still fully usable if driven with care.
By experience I know that typically in 25-30 age the tyre will get a failure in the fabrics causing first bumping and eventually a tyre failure.
I've never said that at the fifth birthday there is a kinf of click, before a tire is good, after is bad. As you rightly say, tires just change, I only set at 5 years my personal limit for a "daily careless usage" even if my two set (summer and winter) usually get fully worn before. I also drove old cars with old tires, they can work but you have to remember that the grip on wet weather won't never be comparable to a new tire grip.
you would certainly have to drive on the edge to notice any possible difference.
Yes, true, sometimes I like to do it, it helps me to "feel the car". It must be said that in Italy temperatures can vary much more than in UK, the same car can find 50° during summer and -15°C during winter (at the ground), so it's easier to feel differences in the behaviour even in dry roads. It's well known that very cold weather is heavy for summer tires because they become hard, but also very hot grounds are heavy because the rubber becomes too soft and jelly, and the grip suffers.