Most factors will purchase invoices that are slightly past due. If you have net 30 payment terms a factor may be willing to purchase an invoice that is 40-45 days old. The factoring company would need to verify that the invoice has not been paid yet.
If your invoice is significantly past due, for example, 80-90 days it is then considered a defaulted invoice. A factoring company will purchase a slow-paying invoice from a creditworthy company, but not a defaulted invoice. Most receivable financing agreements have a 90-day clause that states an invoice must be paid by your customer in 90 days or the invoice is sold back to the client.
Factoring can improve your cash flow especially with slow paying customers. If your customers are creditworthy a factor can purchase the invoice. Many large companies have solid credit scores but pay slowly to help their own cash flow. Invoices from larger credit worthy companies that are net 60 are good candidates for factoring.