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Background The UK Government’s ambition to remove barriers to opportunity and create the healthiest generation of children ever highlights the need to understand what enables young people to thrive as they enter adulthood. This study introduces the Index of Positive Adult Transition (IPAT), a composite measure capturing educational attainment, good mental and physical health, healthy weight, and absence of criminal justice involvement. Together, these domains represent the foundations of lifelong wellbeing. We examine how persistent family-level adversities, including poverty, parental mental illness, parental alcohol use, and domestic violence, affect adolescents’ ability to achieve this multidimensional profile of positive transition.Methods Data were drawn from 9,316 participants in the UK Millennium Cohort Study. Group-based multi-trajectory modelling identified longitudinal patterns of family adversity from infancy to adolescence. Multivariable logistic regression estimated associations between adversity trajectories and IPAT achievement at age 17. Population attributable fractions (PAFs) quantified the proportion of IPAT non-achievement attributable to adversity exposure, compared with a low-adversity reference group.Results Overall, 31% of adolescents achieved IPAT. In the low-adversity group, 45% achieved IPAT, compared with 18% among those exposed to persistent poverty and poor parental mental health. Relative to the low-adversity group, adjusted odds of achieving IPAT were 36% lower for persistent poor parental mental health (aOR 0.64, 95% CI 0.52–0.78), 54% lower for persistent poverty (aOR 0.46, 95% CI 0.38–0.57), and 66% lower for co-occurring adversities (aOR 0.34, 95% CI 0.26–0.43). PAFs indicated that 25.4% (95% CI 20.6–30.3) of IPAT non-achievement was attributable to adversity, with persistent poverty alone accounting for 10.2% (95% CI 6.8–13.7).Conclusion Persistent poverty and poor parental mental health significantly constrain young people’s capacity to achieve a positive and healthy transition to adulthood. Reducing these risks through coordinated, family-centred policies could advance the government’s goal of a generation equipped for lifelong wellbeing.