Aged Coffee

Aged coffee is held as green coffee in producing countries for an extended time (usually 2-3 years) to allow natural changes to occur. These changes typically include a muting of acidity and improved body. Some defects can also be muted, but because of the cost of holding the green coffee for an extended period of time the beans selected for aging is of exceptional quality. During the aging process the coffee is rotated to allow the coffee to breath and evenly change. 

Aging coffee is a risky proposition for the person aging the coffee since there is not a guarantee that even with utmost care the aged coffee will come out as hoped. The producer can end up with a large quantity of old, bad coffee. 
Aged coffee and old coffee are not the same thing. Old coffee will taste boring and lifeless while aged coffee actually improves (well improves is subjective if you like the flavor it is an improvement) with age. 

Aged coffee will typically start with a coffee that is high in body and low in acidity. The idea is to bring flavor out not create a new flavor. A high acidity coffee will in all likelihood just be old coffee even with great processing technique.