I use mapping and age trajectories of advanced cognitive skills to better understand why these skills are more prevalent in some local areas than in others. The study begins by explaining what advanced cognitive skills are. It offers a nonspecialist’s review of recent brain science that indicates that adolescence is the key period for the development of advanced cognitive skills. The paper considers three main explanations for why the prevalence of advanced cognitive skills varies substantially across US counties. Is it early childhood factors which could generate endogenous responses that are important later when advanced cognitive skills are developing? Is it factors whose influence is greatest during adolescence—the period when brain science argues that experience would most directly affect advanced cognitive skills? If so, adolescence is indeed the age of opportunity but also risk. Is the variation among counties explained by migration of individuals toward areas where other people have advanced cognitive skills similar to their own? Evidence based on cognitive skill trajectories, maps at different ages, and longitudinal regressions suggests that all three of these explanations play a role in generating areas where advanced cognitive skills are prevalent and areas where they are not—advanced cognitive skill deserts.