All beef is aged.
Beef aging does not pertain to the age of the cattle but instead refers to the amount of time the meat has been stored and refrigerated after slaughter. We age beef because freshly-slaughtered beef tastes metallic and less "beefy". Frankly, it does not taste very good.
The better the aging process, the better the meat usually is. High-end steakhouses and butchers such as us invest significant time and money into properly and consistently aging beef. However, even your average supermarket beef has been aged for at-least a week.
There are two methodologies to age beef: Wet-Aging and Dry-Aging.