cam belt replacment

I've just been told by my Audi dealer that my A6 is due for a new cam belt because it's 5 years old, the service book says it should be changed at 133,000 miles and there is no mention of age, now the thing is I'm retired and drive less than 4000 miles a year, and so the car has just 19,200 miles on the clock, I thought I would never need to change it, so hell of a shock when they told me it's due in one month at a cost of £455.
Any thoughts ?