Usually seven years is a limit to what bill collectors can go back on. You can contest the collection and refuse to pay if you know you are not responsible. Or work out a payment plan. Either way you are I'n the driver seat. They may not want to collect if you show them you can't be intimidated
Most debts come off of your credit report after four years, except taxes, they last seven. If they are coming after you now, they are trying a hail Mary pass. If you respond now the clock can start all over again. Even if you respond to tell them you aren't going to pay, they can start the collection clock all over again. Do not let them intimidate you into a response, and they will go away.