http://www.britannica.com/eb/article-13356/human-intelligence

Also significant are findings that heritability can differ between ethnic and racial groups, as well as across time within a single group; that is, the extent to which genes versus environment matter in IQ depends on many factors, including socioeconomic class. Moreover, the psychologist Robert Plomin and others have found that evidence of the heritability of intelligence increases with age; this suggests that, as a person grows older, genetic factors become a more important determinant of intelligence, while environmental factors become less important.