You can always be sued for a debt that you don't pay. The statute of limitations may be a defense to the lawsuit, but unless you affirmatively assert this defense, it will not prevent the collector from suing or obtaining a judgment.
The statute of limitations varies widely from state to state. Since most states post their laws on the state website, you may wish to review state law to determine how much time must pass before you can use the statute of limitations to defend yourself in a lawsuit.
Hope this perpective helps!
I agree with the previous answer and would start with an inquiry as to the Statute of Limitations in your jurisdiction. This may prove to be the simplest resolution as it would make the debt legally unenforceable.
Either way, until you know that it is enforceable, do not admit that you owe the debt now, that you ever owed the debt, or that you have any means or intention of paying it. Tell them that you are researching the validity of their claim and suggest they do the same.
Jared L. Haddock