You can’t go by not seeing any cracking. On our Honda, the second set of Michelin MXV4 tires that were only about 4 years old failed with sidewall blowouts and no evidence of checking or cracking. First a front tire blew out on my wife, the tire store blamed it on hitting a pothole or curb. So we replaced both front tires. A couple weeks later, I was sitting at a stop sing waiting for traffic to clear when I heard on of the rear tires blow out suddenly. This time the tire store honored the warrantee.
BTW both time the tire that blew was on the drivers side of the car, no curb abrasion.
Bottom line, time for new tires. Also, I still use Michelin MXV4 tires on that Honda, the current set is about 6 years old with no issues, but they will get changed at the end of this summer.
I would replace ALL the tires. The best ones are 7 years old - and as I said in the thread pointed out, this is all about risk - and limits between 6 and 10 years have been pointed out by various parties.
A Corvette would be a horrible thing to have wrecked just to save the price of tires.
Oh and one last thought: Lack of cracks is NOT a guarantee that the tire is OK to use. Cracks are caused by the state of the material AND the amount of flexing. Lack of flexing - like when a vehicle is stored - can hide a tire that is bad.
Rubber age ?
Think of your old shoes.
Now go to your closet and get that old pair of rubber soled slip-on boat shoes.
Back when they were new you roll the toe back to the heel in your hand almost in a ball.
but NOW ??
Roll that toe back and feel and watch that old sole now !
You may see it crack now where there was no visual cracks before.
Which would you expect to get good grip from if you had both , new and old , shoes in your closet ?
Which would you choose to wear best ?
That’s how your old tires feel as you begin flexing them now after a long winter’s nap ( ok 13 winter’s naps )
Flexing occurs 100% constantly with every rotation, not just with a pot hole or two. From extended on the top of the rotation to squashed at the bottom, this is repaeated with every roll.
Flexing creates heat.
Can they take it ?
Are you a gambling man ?
Can’t afford to fix it ?
-----you can’t afford NOT to !