Sounds to me that you already have your answer. You've essentially determined your own issue: you need to accurate derive/store the effective date of the credits/payments instead of the POSTING date. By trying to run aging against the actual date posted - you will never get the result you want.

Do you have a "PaymentsApplied" (or CreditsApplied) table? Because it sounds to me that you'd need one to do this accurately and efficiently. This would "map" the payment into the various invoice(s) it is being used to pay off/reduce as might be the case.