Old rubber "dries up" (actually it's oxidative polymerization) and cracks. As long as it's surface cracks, it only means you need to get out and ride more to wear the tread out before the cracks grow.

FWIW, my pre-bought tires stay in a plastic bag under the bed; climate controlled, dark, and protected from ozone. I really need to start labelling them with purchase date, some of them have been around for a while. But I can rarely (if ever) correlate age and flaws.