Agree with p6baseunit. You can't really tell the age of a cambelt just by looking at it, though you can of course gauge an idea of what state it is in by mark one eyeball. If there is any sign of water staining from waterpump leakage on the belt, it and the waterpump need changing absolutely asap, along with tensioner/roller if applicable. Also, any visible cracking, it needs changing, but in any case, I wouldn't go beyond 5 years. Ordinarily, under ideal storage conditions, cambelts have a shelf life of 8 years, assuming they have been stored correctly. You will always however have the odd person who will emphatically state they've had the same cambelt on their car for 10 or 20 years or have 'reused' a cambelt they thought looked 'Ok'. I say good luck to 'em!
What I will never understand are mechanics who don't recommend a waterpump/tensioner change when the cambelt is done or customers who think they're somehow doing themselves a favour by refusing to have the waterpump changed at the same time. It's total false economy not to have the waterpump changed at the same time. It usually adds about £40 to the total cost of the job, but can quite literally save hundreds by preventing a wrecked engine.