The statute of limitations normally starts to run at the
later of (a) the last transaction date, (b) the last date a payment was made, or (c) the last date a bill was issued following the last transaction on the account. The specifics can vary by the type of debt.
For details go
here, then click "Colorado Revised Statutes", then "TITLE 13 COURTS AND COURT PROCEDURE", and then "LIMITATION OF ACTIONS".