Not long had my cambelt/waterpump changed out. In my recent experience of this (mine is 2009 with 30,000miles), it's recommended to be changed after reaching either 5 years or some mileage limit (no idea). The problem in the former is that the rubber component of the belt deteriorates with time (whether used extensively or not), causing it eventually to become compromised, potentially leading to a catastrophic failure.
It's apparently the same with tyres that barely get used but should be checked regularly due to the same problem with the nature of rubber itself. The tyres on the rear of my car are factory (2009) rubber with a good 5mm, so perfectly legal, but what is evident is that they don't look so 'fresh' as the newer tyres on the front.