This study analyzes how changes in Korean young adults’ pathways to adulthood affect individuals, families, and society, and sets out implications for population and social policy. The key findings are as follows. First, poverty experience and low educational attainment delay or stall transitions to employment, home-leaving, and marriage, reinforcing cumulative disadvantage. Second, delayed or unrealized transitions are, on average, associated with poorer mental health. Third, the timing of employment and marriage significantly shapes subsequent income and fertility. Fourth, for parents, financial burdens increase, while emotional effects are mixed. At the macro level, a “delayed transition trap” may trigger a vicious cycle across demographic, economic, and social domains. Policywise, the study recommends reframing youth policy in a life-course perspective, expanding early interventions to the child and adolescent stages, and strengthening integrated supports for housing, employment security, and education. In particular, the conclusion proposes securing transition supports as entitlements through a Youth Backpack (Comprehensive Youth Activity Account) and a participation-income-linked Job Guarantee that provides quality employment in the care and climate domains.