I just mounted a set of new tires on a bike last week, and the sidewalls had small cracks. Unusual, slightly unsightly, but not alarming. The rubber still seemed quite supple.

My personal experience is here in Arizona, which is a hostile environment for just about any polymer. I'll use tires until the tread wears through, the cords separate (which typically happens under the tread and not on the sidewall), or the bead tears & blows. The bead tears & the cord separations seem unrelated to sidewall cracking. Sometimes I'll run into an even older tire where the tread delaminates while riding, and this is associated with sidewall cracking, but the tire is sufficiently "experienced" that they are both just symptoms of age.

To get directly to the issue: I have never in 40+ years of riding had a sidewall failure or blowout directly attributable to a sidewall crack. All the sidewall blowouts have been bead failures or accidental brake wear at the rim lip.