Background: The sharp decline in mental health among younger generations has become a key global health crisis of the twenty-first century. While often linked to short-term life-cycle stages or common period effects, emerging evidence indicates distinct “generational scarring” among Millennials and Gen Z (Generation Z). The causes of this decline in mental health are still debated, with theories ranging from economic insecurity to social fragmentation.Objective: The present study aims to disentangle Age, Period, and Cohort (APC) effects to determine whether the youth mental health crisis is a function of developmental stage or structural cohort disadvantage. In particular, it investigates the differential roles of economic capital, social conditions, and social networks in shaping mental health trajectories in the United Kingdom and Australia.Methods: We analyzed longitudinal data from the Understanding Society survey (UK) and the Household, Income and Labor Dynamics in Australia (HILDA) survey (Australia). We employed generalized additive models (GAMs) to visualize trajectories and Hierarchical Age Period and Cohort models to estimate what explains generational differences in mental health.Results: Cohort variance exceeded period variance in both countries, indicating that youth mental health decline is primarily generational rather than period driven. Gen Z entered adulthood with significantly lower mental health baselines than earlier cohorts in both countries, although Australian Gen Z showed a U-shaped recovery after age 20 while UK Gen Z showed no recovery. In the United Kingdom, the Gen Z deficit was almost fully mediated by perceived social isolation, indicating a crisis of connection, whereas the Millennial deficit persisted net of all economic and social controls, consistent with lasting scarring from the post-2008 austerity period. In Australia, standard economic, family, and social support variables failed to resolve the generational mental health gap.Conclusion: The drivers of youth mental health decline are structurally distinct between Australia and the UK. The United Kingdom requires investment in social infrastructure and community repair, while the persistent Millennial scar signals lasting damage from formative-years adversity. In Australia, the crisis is rooted in restricted occupational agency, requiring labor market reforms that enhance autonomy for younger workers. Public health responses must move beyond a uniform medical model of individual pathology toward context-specific structural interventions that treat social connection and agency as population-level determinants of mental health.