You can make notes on the job through set-up Job.  Then you'd have to add this field to your report through report designer so it will print.

I like the idea of creating notes, which would require updating your report to include that information.

However, I would recommend making the notes on the Invoice as the note would be specific to the Invoice and when the Invoice drops off the aging report so would the note.  If you make notes at the Job level, they will either always be there or you will need to remove the notes from the Job when they are no longer needed.  You can access your Invoice through AR > Tasks > Adjust Receivables and select and Edit the desired Invoice.  You will need to enter an Effective Date (the date you spoke with the customer?), and can then click on the paperclip to add your notes.

Additionally, you can add notes on the Customer record, but I don't think that will help in this case.

You add notes, by accessing the desired Record (whether that is the Job or Invoice) and clicking on either a Paperclip icon or an Attachment button.

I save the aging as a Adobe PDF (by changing the Printer Set Up to Adobe PDF) and then select Tools / Comment function on the PDF to add notes.