Discs wear down just like brake-pads, they'll eventually need replacing. To gauge this they need to be measured accurately as well as visually inspected for pitting or other damage. They are not expensive, simple to remove and replace as they are usually held in place on the wheel-hub by a single screw, with the wheel-bolts doing the heavy clamping-work. The wheel bolts/studs make a big sandwich - wheel, brake-disc, hub. 8 years is a pretty amazing lifespan though.