This crops up time and time again. Obviously age, as well as how many times it's gone round in circles will affect the belt. I don't know who came up with the magic number of five years but I've heard it recommended at that by six different people now. Some do it, some don't.. it's your peace of mind at the end of the day.
A trickier point is the water pump, which (on mk4's at least) is usually changed along with the cambelt at around the 65k mark, because a) with the belt/tensioner off you have very easy access to the pump, so no labour is charged (just the ?20-ish for the pump) and b) the internals of the water pump have a habit of breaking up at about 70k - 80k.
Personally, I'd get the belt and tensioner, and the pump too changed as VW recommended. I'd be gutted if 5000 miles after paying for the belt I have to pay another massive labour charge to get at the water pump !
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