Legal Question in Credit and Debt Law in Pennsylvania

You leave out a lot of facts. While you co-signed a lease 10 years ago, I assume everyone involved was not in cryogenic sleep. So what happened in the 10 years? Was the lease renewed? Did the tenant move? What did the lease say about renewal?

Generally, if a tenant holds over under an old lease then the lease you co-signed would still be valid assuming that the tenant is at the same place. This was a poor way to do things by the landlord. I can't believe that the landlord never bothered to have the tenant sign a new lease.

However, this is a landlord tenant question, not a debt collection issue. You need to ask a landlord-tenant lawyer. I would take a wait and see approach and see if you are sued by the landlord. If the landlord is this incompetent, then I would argue to the judge that just because you co-signed a lease 10 years ago for the tenant should not equate to you being on the hook for the rest of your life for this tenant and that you should not be held liable as a co-signer.