Matt -
The DueDate is just coming from the DueDate field of the statement charge. All statement charges ("Charge" table in Method) have the ability to have a Due Date entered and this is what the portal is showing.
If instead you want it to pull a field from the Customer screen that you are arbitraily setting, that's a totally different thing. Customer's don't have due dates, their transactions do. So in your case you should make sure that all the statement charges you enter are set to be the 15th day of the following month. You can customize the Statment Charges screen to do this for you automatically using actions. Keep in mind if it gets tricky, you can pay us to customize it for you.
I also suggest looking into Method Field Services. You can just schedule your routes so that the jobs show up on a calendar or work order list, and then at the end of the month create invoices, and ALL jobs will turn into an invoice with a due date for you. Less work for you.
Showing aging - I'm sure it is possible. I haven't tried it. The data is all there now in the Transactions table. Really you need to know the date (for the age) and the balance remaining, which we have. I think it would be tricky to get and require some good Method customization experience to get a report, screen or grid to show it.
Paul
Paul