No need to wonder. There's a test for this.

On a relatively new tire, depress the tread with your thumb and note how pliable and soft it is. This is how a tire should feel.

Repeat this test on old tires in question. If there's a big difference, if hardness and brittleness are detected, the tires are too old. It's time to replace them with new tires no matter how much tread is left and even if there is no visible cracking.

I have a Wide Oval tire that's almost 50 years old, and it's not cracked, probably because it was a spare kept out of sunlight. It's rubber is so hard I could not dismount it myself using a 12-ton press. Had to take it to a tire store.