It's not just what you have, but when: The role and timing of developmental assets in the mental health of young adults.

As the mental health of young adults is declining, finding effective strategies to support them is crucial. Young people who possess, or are exposed to, more developmental assets report higher well-being and fewer mental problems. Developmental assets are promotive factors that include both individual strengths, like strong social skills and positive identity, and external resources, like supportive relationships and safe living conditions. However, no longitudinal study has yet examined when in development the presence of developmental assets is most impactful. This study aimed to identify which life-course hypotheses-sensitive period, accumulation, or recency-best explain the associations between developmental assets across childhood and adolescence and mental health in emerging adulthood. Data were drawn from 1344 White, French-speaking participants of the Quebec Longitudinal Study of Child Development (57% female), a population-based cohort of children born in the Province of Quebec in 1997/1998 and followed up until now. Developmental assets, measured as both broad dimensions (internal and external) and specific categories (e.g., social competencies, support), were assessed across development from ages 5 (2003) to 17 years (2015) using reports from mothers, teachers, interviewers, and the youth themselves. Self-reports of life satisfaction, happiness, and symptoms of anxiety and depression were collected twice, at ages 19 (2017) and 22 years (2020). Although all effect sizes were small, findings from the Structured Life-Course Modeling Approach showed that both internal and external assets present later in adolescence were most beneficial in supporting life satisfaction, thereby supporting the recency hypothesis. In contrast, a sensitive period hypothesis best explained the associations between external assets and both anxiety and depression symptoms, with assets present during the transition to secondary school (age 12 years) showing the strongest associations. When examining specific asset categories, social competencies, positive identity, support from families, schools, and neighborhoods, and safety emerged as especially salient for later mental health. This study highlights the importance of understanding when and which assets matter most to inform more developmentally targeted approaches promoting youth well-being.