If you have processed a payment on account to a customers account at some point in time you will need to match this payment with the invoice/invoices.
It could be that the customer has paid you in advance, you have processed the payment and then at a later date processed the invoice.
You can process a payment on account by leaving the invoice box blank when you process the payment.
Until the payment and the invoice have been linked together they will both remain as Open Items on the customer ledgercard and will appear on the aged debtors report.
To clear these items go to Sales > Customers > locate your customer > click on the far right icon Open items this will display a list of invoices/credit notes and payments which have yet to be matched.
Tick the box to the left of the transactions you wish to match, click Match selected and confirm. These entries will now be matched and by doing so will be removed from your aged debtor report.