An A/R aging groups open customer balances by days past due as of a selected date. Use the due date, not the invoice date. An invoice due today is current. Payments and applied credits reduce the open amount before it is placed in a bucket. Aging helps focus collection work but does not itself prove that a balance is uncollectible.
Under the allowance method, estimate expected credit losses with Bad Debt Expense and an allowance contra asset. A specific write-off then debits the allowance and credits A/R, so it does not create a second expense. To recover a previously written-off amount, restore the receivable and allowance before recording collection. Customer detail must remain attached to receivable entries.