This also gets at the fact that IQ is closely linked to the family's income level and the mother's (and probably father's) educational level. IQ isn't simply a set-in-stone number that cannot be changed up or down.
Studies have shown that babies in wealthy families are exposed to more words and more complex words during development than babies in poor families. Maybe that alone would affect your verbal IQ tested at age 5.
I guess that the question then becomes whether knowing more words due to enrichment actually indicates that you are more intelligent. I view intelligence as the capacity to learn -- like Bostonian mentions, absorbing more. I suppose that's why IQ isn't viewed as "set," per se in early childhood b/c environmental impacts matter more the younger you are. I agree that IQ isn't set in stone, but I'd imagine that a range at which you will eventually land once the environmental factors settle out is fairly set. I don't, personally, believe that you can take someone who is wired as average and change the brain wiring so dramatically as to make that person way out of the norm either way (of course, save for a brain injury or some other injury like prenatal drug exposure).
