The developmental perspective applied to psychopathology has led to the concept of early-and late-onset disorders. This study explores the application of the early- and late-onset concepts of antisocial behavior to physical aggression. Are there two categories of chronically physically violent adolescents: those who are physically aggressive throughout childhood and those who start being physically aggressive during adolescence? The estimation of developmental trajectories for repeated measures of two different response variables—physical aggression in childhood as measured by teacher reports and physical aggression in adolescence as measured by self-reported violent delinquency—is achieved with a semiparametric, group-based method. This new method is applied to a large sample of males from Montreal who have been assessed repeatedly since kindergarten. Several salient findings emerge from the analysis. First, we find considerable change in the levels of childhood and adolescent physical aggression. Thus, there is little evidence of stability of behavior in an absolute sense. A second key finding concerns the connection of childhood aggression to adolescent aggression. Boys with higher childhood physical aggression trajectories are far more likely to transition to a higher-level adolescent aggression trajectory than boys from lower childhood physical aggression trajectories. However, for all childhood physical aggression trajectory levels the modal transition is to a relatively low-level adolescent aggression trajectory. Third, we find little evidence of “late onset” of high-level physical aggression. Specifically, the joint trajectory analysis finds no evidence of transition from a low physical aggression trajectory in childhood to a high trajectory in adolescence.