I would say the younger you test the more likely score will be lower. My son took the wpssi at age 3 and wisc at age 8 and there was a big increase. At age three he could barely focus and sit still, let alone do the writing task.
However, I have heard of children leveling out as they get older. Ex. Child scores very high at age 5 but by age 9 they are only slightly above average.