In
this downloadable document [Word format], Legal Services of Northern Virginia indicates that the statute of limitations is three years, and argues that it might be possible to assert that it is actually two years under certain circumstances. (In your case, it doesn't seem to matter - the debt is more than three years old.) Note that in actions on an open account, the statute of limitations starts to run from the
later of the last payment or last charge for goods or services rendered on the account.
Note that if you pay even a cent toward the balance after expiration of the statute of limitations, you could be considered to have renewed the entire debt with the limitations period starting over again from the date of your new payment.
Take a look at the sample FDCPA letters
here and
here, and consider sending the collection agency a letter asserting that the debt has expired and that they should not contact you any more.