Calculate aging over time

Where can I find the information I need to calculate customer aging over time? Usually when I get stuck I can copy something one of my predecessors did, but no one knows of any report like this previously. 

I want to be able to show a sales person, for instance, that their receivables for an account are increasing over time. Eg., when viewing the report in June, they'd be able to see in January that their net30 for a customer was $1000, February it was $1500, March.... etc etc. But, I'm stuck as to what would be my starting point to establish initial balance etc. I would assume it's something like transaction history, where to find quantity at a point in time I would calculate the flowing river after Jan 1 of each year... But again, stuck as to where to start... Maybe I need to establish that beginning yearly balance in a table and calculate from there to avoid the queries being too monstrous. Hope I'm not over thinking this.