Using the simplest answer concept, then I would suggest that the vendor's invoice is not marked paid in the program.

However, if it just recently started (we don't know how long it's been showing up), then it is possible (but not probable) that someone posted a manual adjustment to the vendor account.

Assuming there is no corruption in your file, a logical reason this might happen is that the vendor has been marked Inactive and the Include Inactive Vendors has been turned off for the last 16 years.  Now someone has turned it on and now you see the vendor.  This also is unlikely because someone would have noticed the Supplier Aged reports don't match the Balance Sheet Accounts Payable account balance over that time period.

Without being able to see the file directly, it's hard to provide any other reasons for this behaviour.