Aging is the length of time a balance has been outstanding and is useful for billling.
Aging is also automatically calculated when users:
By default, all charges within a family are sorted into four aging categories based on the date of the charge. The categories are 0-30, 31-60, 61-90, and over 90 and represent the number of days the remaining balance of a charge has been outstanding. Then, the sum of all credits for the entire family history is applied to the oldest category with a remaining balance. If the total credits were not sufficient to cover the total charges, categories will still contain amounts due.
Credits are applied to the oldest debts first in order to give the family as much credit as possible. This way, if you see a family has a balance in the over 90 category, you might be more comfortable moving the guarantor account to precollections. However, credits (e.g. adjustments, pay plan charges) attached to procedures can be applied to the procedure balance instead of the oldest family balance. See the Preference, LIFO: Transactions attached to a procedure offset each other before aging.