Q: What is aged beef?
A: Aged beef is beef that has been stored in a controlled, refrigerated environment to ultimately increase the overall flavor of the beef. During this "drying," the beef goes through an enzymatic process, changing the flavor, tenderness and weight of the beef. Usually, the longer it is aged the more tender it gets, up to a point. Beef can be aged anywhere from 7 to 14 days (the average time being 11 days). The more marbling (or fat) in the beef, the better it ages, since fat helps retain moisture and flavor in the beef. Therefore, fattier cuts of beef, such as porterhouse, and higher grades, like prime, are ideal for aging.
Cooking : Culinary Q&A : Aged beef : Food Network
Kroger use to age 30 days. This info is somewhat misleading, as it says the carcass can lose up to 40% of it's weight. That's just totally off the wall.