Despite underlying regularities in the age profile of migration, there is mounting evidence of cross-national variations in the ages at which migration occurs. Explanations for these differences have variously been sought by reference to cultural, social, and economic factors, and through analysis of reasons for moving. There is also a growing body of work linking migration events to particular transitions in the life course. We set out a conceptual framework that links contextual factors to the age structure of migration through life-course transitions that act as proximate determinants of the age at migration. We propose metrics to capture the prevalence, timing, and spread of four key life-course transitions: education completion, labor force entry, union formation, and first childbearing. We then seek to quantitatively establish the link between these indicators and the age and intensity of internal migration at its peak for a global sample of 27 countries. Correlation and factor analysis reveal substantial diversity in the timing and spread of transitions to adult roles, and show that cross-national differences in the age profile of migration closely parallel variations in the age structure of the life course for over two-thirds of countries. Migration age profiles are aligned with transitions to adulthood for both sexes but most strongly among women.