I’ve seen this phenomenon a lot lately and I wonder if recent production methods are producing a tire more susceptible to this - during the last several years, I have tires that aren’t old at all that are getting small cracks in the sidewalls. I’m pretty sure a lot of this is due to UV damage, on my vehicles that spend all their time outside.
I think tire dressing helps to make the rubber last longer, but I’m not good about keeping it applied. I don’t know how to describe how to look at the tires, but I think it’s gotta be pretty severe dry rot before it becomes a safety issue.