Little is known about how key aspects of parental migration or childrearing history affect social development across children from immigrant families. Relying on data on approximately 6,400 children from the Early Childhood Longitudinal Study-Birth Cohort, analyses assessed the role of mother's age at migration on children's social development in the United States (sociability and problem behaviors). Consistent with models of divergent adaptation and assimilation, the relation between age at arrival and children's social development is not linear. Parenting practices, observed when children were approximately 24 months of age, partially mediated the relation between mother's age at arrival and children's social development reported at approximate age 48 months, particularly in the case of mothers who arrived as adults. (Contains 4 tables.)