Depression is a leading cause of disability. The timing and persistence of depression may be differentially associated with long-term mental health and psychosocial outcomes. To examine if depression symptoms during early and middle childhood and adolescence and persistent depression symptoms are associated with impaired young adult outcomes independent of early risk factors. Data for this prospective, longitudinal cohort study were from the Québec Longitudinal Study of Child Development, a representative population-based Canadian birth cohort. The cohort consists of infants born from October 1, 1997, to July 31, 1998. This is an ongoing study; data are collected annually or every 2 years and include those ages 5 months to 21 years. The end date for the data in this study was June 30, 2019, and data analyses were performed from October 4, 2022, to January 3, 2024. Depression symptoms were assessed using maternal reports in early childhood (ages 1.5 to 6 years) from 1999 to 2004, teacher reports in middle childhood (ages 7 to 12 years) from 2005 to 2010, and self-reports in adolescence (ages 13 to 17 years) from 2011 to 2015. The primary outcome was depression symptoms at age 20 years, and secondary outcomes were indicators of psychosocial functioning (binge drinking; perceived stress; not being in education, employment, or training; social support; and experiencing online harrasment) at age 21 years. All outcomes were self-reported. Adult outcomes were reported by participants at ages 20 and 21 years from 2017 to 2019. Risk factors assessed when children were aged 5 months old were considered as covariates to assess the independent associations of childhood and adolescent depression symptoms with adult outcomes. The cohort consisted of 2120 infants. The analytic sample size varied from 1118 to 1254 participants across outcomes (56.85% to 57.96% female). Concerning the primary outcome, adjusting for early risk factors and multiple testing, depression symptoms during adolescence were associated with higher levels of depression symptoms (β, 1.08 [95% CI, 0.84-1.32]; P < .001 unadjusted and Bonferroni adjusted) in young adulthood. Concerning the secondary outcomes, depression symptoms in adolescence were only associated with perceived stress (β, 3.63 [95% CI, 2.66-4.60]; P < .001 unadjusted and Bonferroni adjusted), while both middle-childhood (β, -1.58 [95% CI, -2.65 to -0.51]; P = .003 unadjusted and P < .001 Bonferroni adjusted) and adolescent (β, -1.97 [95% CI, -2.53 to -1.41]; P < .001 unadjusted and Bonferroni adjusted) depression symptoms were associated with lower levels of social support. There were no associations for binge drinking; not being in education, employment, or training; or experiencing online harrasment. In this cohort study of Canadian children and adolescents, childhood and adolescent depression symptoms were associated with impaired adult psychosocial functioning. Interventions should aim to screen and monitor children and adolescents for depression to inform policymaking regarding young adult mental health and psychosocial outcomes.Read less <