Hi Mark,
I have used Power Query, Conditional column to determine if the invoice is unpaid based on the logic you stated above. I then create two measures, Unpaid invoices and Unpaid Invoices by Aging Groups.
My measure for Aging Groups follows the example by Sam.
Have a look at the attached file, in page 1, table on right. Is this working how your expect?
AR Aging Example Kim.pbix (996.6 KB)