Policy researchers have long known that the impacts of policies and programs differ across individuals. Recent efforts have been directed toward quantifying the scope of treatment effect heterogeneity. But little energy has been expended to conceptualize and test hypotheses about the nature of such treatment effect heterogeneity. With regard to human capital skill and behavior interventions directed at children and adolescents, we posit that a human development perspective provides a useful framework for understanding major sources of treatment effect heterogeneity. The cognitive and socio-emotional development of children and adolescents follows a predictable pattern of stages in which windows for profitable interventions widen or narrow. Moreover, there are predictably persistent differences in individual characteristics such as temperament and family circumstances (e.g., socioeconomic status) that can help explain within-stage variation in intervention impacts. We join others in arguing that key to successful human capital interventions is the “fit” between a given policy intervention and the developmental stage and context of the child or youth participants. Our framework generates many concrete predictions regarding the kinds of human capital interventions that are likely to work for which kinds of children and youth and when during their development.