Our legal tradition has always distinguished between children and adults and justified those distinctions in developmental terms. Only relatively recently, however, has that development been extensively studied by psychologists and still more recently, by neuroscientists. Conventional wisdom among children’s rights scholars holds that law should take account of this growing body of science and social science, and should assign rights and responsibilities that more accurately reflect the assessments of children’s capacities documented in the scientific research. In this Article I will argue that a more sophisticated understanding of child development counsels against an approach to children’s law that treats children’s capacities at certain ages as ascertainable and fixed. Instead, the law should recognize the contingent nature of children’s capacities and, as important, identities, and the role law inevitably plays in fostering or thwarting children’s growth.