In this article, the authors analyze cross-national variations in how the category of ‘juvenile’ is defined in criminal law and policy. The authors purposively selected the cases of Argentina, Belize, England/Wales, and Finland to maximize differences in the boundaries of the Minimum Age of Criminal Responsibility and the Age of Criminal Majority. Legal analysis identified two key factors: (a) the presence or absence of a distinct juvenile justice system, and (b) the stability (or fluctuation) of youth justice laws and age boundaries. These axes of difference and their various configurations across cases have broader implications for advancing children’s rights.