By ANNIE LOWREY | New York Times – Fri, Sep 21, 2012 6:00 AM EDT A new study by researchers at the Brookings Institution shows that about two in three Americans achieves a middle-class lifestyle by middle age - and delves deeply into who makes it there and how. Isabel V. Sawhill, Scott Winship and Kerry Searle Grannis tackled the question of why some children make it to the middle class and others do not, studying criteria that tend to be indicative of later economic success and examining how race, gender and family income come into play.