The marbling (intramuscular fat) develops while the steer is alive, not during aging. Alton explains the dry aging process this way.
"Like balsamic vinegar and hard cheese, beef improves with age. That's because like vinegar and cheese, beef is mostly water. In fact, about eight and a half pounds of this ten and a half pound roast is indeed H2O, a substance not famous for its flavor. However, in just a few days we can eliminate enough of that water to seriously intensify the flavor of the meat. This is going to take time. But that's okay, because meanwhile, enzymes inside the meat will be hard at work breaking down connective tissue, and that means a more tender piece of meat."
Fancy restaurants will have cold rooms where the meat is aged for weeks, or even months (some top steakhouses will age beef for four months!), but if you want, you can try dry aging a few steaks in a plastic tub with a perforated top in a refrigerator chilled between 36 and 38 degrees Farenheit, with around 50% humidity. Try it for 72 hours and see how it turns out.