It is well-known that childbearing is associated with age at migration, but most research has focused on foreign-born women who migrated as adults. Much less is known about male immigrants or immigrants who arrived as children, despite the importance of studying these groups to understand theories of adaptation and socialization. This study addresses these gaps with a case study of Sweden, using longitudinal whole-population data to analyze the role of age at arrival in determining childbearing. The results suggest that age at arrival affects fertility across the childbearing life course, although there is little evidence of critical ages at arrival. These results hold for women and men, particularly for immigrants from higher fertility origins, with more ambiguous results for immigrants from lower fertility origins. The main findings also persist after examining sources of selection and reverse causality using sex-specific family fixed-effects models and separate analyses for specific countries of birth. Therefore, the study provides evidence of an underlying process of childhood socialization, followed by adaptation, that is common for women and men who migrate. Theoretical implications are discussed, including the need for further work on the determinants and mechanisms of adaptation.