Aging 

In the Main Menu, select Tools | Calculate Aging. The following window will appear:

You have a choice between monthly and daily billing.  Billing.  We recommend daily, but you can switch to monthly in Module Setup by checking the option for "Aging calculated monthly instead of daily".  This will have a couple of consequences:
1. When you open a patient Account, the aging will be automatically calculated as of the last calculated date rather than the current date.
2. When you do calculate aging from the window above, you will have to manually change the date to the one that you want. You would typically be doing this on or near the first of each month, and you would change the date to be exactly the first of the month. 

Regardless of which billing method you use, aging will be automatically calculated for all patients in a few places.  You will not see any visual indicator that aging is being calculated because it is very fast.  It happens when creating an Aging Report, running Billing, and running Finance Charges.

There is an overview of Feature Requests associated with the Aging report here.