A larger working-age population supports economic and social development, and many developing countries are currently benefiting from this demographic window. However, internal migration—often age-selective—can create subnational inequalities by redistributing working-age individuals from less developed areas to cities in search of employment and upward social mobility. While many studies have explored the age selectivity of migration, few have examined its impact on subnational age structures, and none have assessed variation within urban areas using globally representative data from the Global South. This study addresses that gap by analyzing how migration affects the working-age population across the urban hierarchy during national urbanization, using 103 microcensuses from 46 developing countries. Findings show that internal migration's impact varies by urbanization level: it generally increases the share of working-age individuals in cities but has the opposite effect in rural areas. In big cities, migration boosts the working-age share, but this effect weakens and eventually reverses in smaller towns and rural areas. These patterns are largely driven by migration among young adults aged 15–24. The study advances a generalizable theory on migration's role in shaping age structures, helping to explain subnational variations in the demographic dividend and supporting more targeted policy planning.