Mechanism-based prevention has transformed developmental psychopathology by shifting the focus from whether interventions work to understanding how they work through the modification of neurobehavioral processes underlying risk. We argue that this framework remains incomplete without equal consideration of when during development interventions are delivered. Because neurobehavioral mechanisms emerge and mature along distinct developmental trajectories, interventions are likely to be most effective when they target mechanisms during periods of heightened developmental plasticity. We propose a framework of mechanism-specific developmental windows of opportunity, in which the timing of intervention is considered alongside mechanistic target selection. Evidence from developmental neuroscience and prevention research suggests that interventions delivered during different developmental periods may capitalize on distinct forms of neurobehavioral plasticity, from broad foundational processes in infancy to more specialized systems that continue maturing throughout childhood and adolescence. We further highlight that environmental context, particularly early adversity, may shape the malleability of these systems and influence responsiveness to intervention. Integrating developmental timing into mechanism-based prevention has the potential to improve intervention precision and alter trajectories of child and adolescent psychopathology.