If you believe the debt is expired, and from what you've told us it presumably would be, consider sending a letter like the one in this thread. The statute of limitations that would apply is normally that of the venue in which you're sued (many states have 'borrowing statutes' that will apply the SOL from the state where the action accrued, but normally only if it's less than or equal to the state's own SOL), but you can cite both.