Newer tire rubber compound may degrade even faster than older tires, as environmentally friendly compounds are increasingly being used.
Generally speaking, tires that are mounted on the car should be replaced within 7-10 years of the tire date code, regardless of distance travelled. Cracking is only one indicator of impending tire failure. Wallowing, directional stability problems, or harsh ride are also indications that your tires are too old and should be replaced. They may still be okay for short distances at low speeds. You really don't want to risk having your tread peel off or separating from its casing at highway speeds.