Customer statements - Customisation
Customize customer statements to include or exclude prepayments, overdue by 30/60/90 days, and any other customizations users would like (i.e. images.)

Hi Lana
There is already a way to customise statements to some extent. You could add a Custom DOCX branding theme, which allows you to download our sample templates for invoices, statements, etc, for you to customise in Microsoft Word.
With the Statement template open in Word, you can add logos and make any other changes you would like, before saving the file and uploading it back into your branding theme in Xero.
You'll then want to select this custom branding theme for your statements. There's more on this here: Create an advanced invoice template (it's the same feature for statements)
If you'd like to try this out before taking action in your own org, you can use the demo company to test it. 🙂
Please note that currently, customer prepayments will only be shown on a customer's Activity statement, not Outstanding-type statements.
You also asked about ageing (30, 60, 90 days). This isn't currently possible, but a possible workaround might be to run the Aged Receivables detail report you can choose how many ageing periods you'd like to show and set the period to '30 days'. Simply Filter the report by Contact and Update to run.