If you want an A/R aging report as of a certain date, you should run the AR Aging report and then apply the As Of date filter. This report is available from the main menu > Reports screen > Aging. This report provides a reliable way of knowing what your A/R balance was as of a specific date.
You can view selective information on this report by applying the relevant filters to it such as the As Of date. You can also apply other filters as needed on the Customize dialog. Check CORE Help Center for details.