cam belt wear can be measured in two ways firstly by mileage as you already said but also the age of the belt is a factor. Alot of manufactors have a recommended time to change regardless of mileage. The belt is made from rubber and will degrade over time. The existing belt is now just over 7 years old so maybe time to change because if it were to break it would cost alot more for an engine rebuild, hope this helps.
Grant Scott
November 2007