The way that collection agencies work, they pass their accounts amongst each other to try to toll the statute of limitations, which in most cases is four years. Contact a local attorney.
A creditor cannot collect on a debt forever. A lawsuit attempting to collect a debt must be commenced within a certain time after the debt becomes due. The law governing the length of time within which a collection lawsuit must be started is called a "staute of limitation". Statues of limitation vary from state to state and among different kinds of debts. You need to ask a lawyer who practices in the state whose law governs your obligation to pay the utility bill whether this bill is too old. Many factors will determine whether this 13 year old bill is too old to collect or not. If the bill is being handled by a collection agency, their attempts to collect may be a violation of the Fair Debt Collection Practices Act (FDCPA) if the bill is too old. Again, you should contact a lawyer who practices in the area of consumer law and/or the FDCPA to deterrmine what your rights may be.
I agree with Cass Weil that a Fair Debt Collection Practices Act (FDCPA) violation may have occurred if the debt is no longer valid and misleading representations have been made by the collection firm. If so, you may be able to seek $1000.00 in damages, in addition to actual damages and your attorney fees. This is a good example of a case that should be taken to an attorney for a free consultation. An attorney should be able to review the written correspondence you have had with the debt collector and discuss the details of your situation to determine whether you have a defense or possible claim against the debt collector.