If it's an older car and not worth much it can cost more to have the cambelt and tensioner replaced than it is to buy another cheap car. Mine was due 15k miles ago but it's a 53 plate car and a nitemare to change it. I'll change the car before the belt. I have a 61 plate Astra but that's worth a lot more so I don't mind spending when it comes to needing replacing,whether that be mileage or age related. The belt will deteriorate over the 5 years so I'd change it even if it had only done half the miles it can do. An easy comparison is tyres, old ones will dry out and crack so will need replacing even if they have only done a few miles, it's the same with rubber timing belts.