After 6 years in England and Wales and 5 years in Scotland. If no payment is made or there has been no communication in that time, then the debt is unenforceable by the court.
If the Company can provide adequate proof they have tried to contact you (i.e. written to you, you've moved house and simply not told them), then the court can enforce it.
To elaborate on the above answer, it is 6 years (or 5years) since the last date you communicated with them, not the age of the debt or the date of the last payment.
Communication includes making a payment or contacting the company via email, telephone, by post or in person.
But don't get the idea this means the debt has been written off. It still exists and you can still be chased for it - it is just that they can't go to Court to enforce it (unless they have already got a Court Judgement before the 6 years is up).