This seems like a paradox to me. On one hand people point to things like the Flynn effect and early interventions as evidence that IQ is malleable. On the other hand the behavioral genetics literature shows that heritability of IQ increases from around 40 percent in childhood to around 80 percent in adulthood while shared environmental influence drops toward zero. If IQ were genuinely malleable you would expect environment to maintain its influence over time rather than fade out. How do researchers reconcile these two patterns?
This is one of the most underappreciated findings in behavioral genetics and the reconciliation actually makes sense once you understand gene environment correlation 
In childhood your environment is largely imposed on you by your parents and circumstances. As you age you increasingly select and create environments that match your genetic propensities. A high IQ child placed in a stimulating environment by their parents gradually seeks out increasingly intellectually demanding experiences on their own. That active gene environment correlation amplifies genetic differences over time rather than dampening them. So the increasing heritability with age does not mean environment stops mattering, it means people increasingly live in environments shaped by their own genetic tendencies. The Flynn effect operates at the population mean level and does not contradict this because it does not compress the distribution or change the rank ordering of individuals.
The fact that heritability increases with age while shared environment fades toward zero is basically the opposite of what you would predict if nurture were the dominant force 
It suggests that as people gain autonomy over their own lives their genetic propensities express more fully rather than less 