The evidence that adult IQ is unrelated to postnatal environment

I've seen this mentioned several times. I don't know how good the evidence base is; we've seen similar results turn out to be based on very little data.

If it's true, it's rather convincing evidence that adult intellectual ability is not much affected by postnatal life, and is probably almost entirely innate.

Searching for Intelligence in Our Genes: Scientific American:

... “Two people with the same genes correlate as much as a person does with himself a year later,” Plomin says. “Identical twins reared apart are almost as similar as identical twins reared together.” But these similarities also take time to emerge. “By the age of 16 these adopted-away children resemble their biological parents’ IQ just as much as kids do who are reared by their biological parents,” Plomin adds...

The corollary is that IQ results are impacted by environment prior to age 16.

I would like to know how good the evidence base for this really is.

On a related topic, we know that the brain is extensively transformed during adolescence. Is this when the innate IQ "reset" occurs?