This article offers a reconsideration of the campaigns to increase the age of consent as constituting an ultimately unsuccessful effort to extend the concept of childhood, and the protection granted to children, so as to include teenage girls, rather than simply an attempt to regulate female sexuality. Purity reformers' concern to redefine childhood in this way reflected a growing attention to physiological and psychological development in the aftermath of the Darwinian revolution. Viewed in this light, children appeared more sharply different in nature from adults, until beyond the end of puberty. That modern understanding of childhood also shaped the enforcement of the age of consent in New York City. Reading prosecutions for statutory rape from the case files of the Manhattan County District Attorney alongside those for child rape reveals that jurors could not fit teenage girls to their older, plastic ideas about childhood, and consequently would not offer them the same protection that they provided to younger girls. Prosecutors responded by employing different charges for the two age groups of girls. Encounters between ordinary New Yorkers and prosecutors thus helped elaborate and dramatize distinctions between teenage girls and younger girls, preparing the ground for the modern notion of adolesence.