My understanding is that there is some regression to the mean until about 10 years old. However, after about 10 the range is typically stable. I have heard of children scoring in the 150's on a WPPSI at 5 and then being tested again at 10 and scoring high 120's low 130's. Primarily because verbal skills were way above average for their ago but then other children caught up, at least in terms of what is measured on an IQ test.