Aged rubber causes problems, depends on your micro climate. Stored inside, outside, sunny, not, hot, etc.
On my Miata, I just looked at the 8 year old tires, they're TOAST. No symptoms last time I looked, but now--
Sidewall crazing, split rubber in the treads, particularly close to the sidewalls (this scares me most--chunky tread cracking). The Nox OEM Michelins did the same, about the same life span too. But, plenty of tread left in both cars, so my cue is do not spend on high mileage tires, buy grippy tires that wear quick. I'll keep these Miata tires for the winter, then put summer treads on in the spring.
On my trailer tires, stored inside, they age rotted at 16 years. If yours look OK, then great. All the more reason to rotate your own tires.