IMPORTANCE Understanding which children in the general population are at greatest risk of poor functional outcomes could improve early screening and intervention strategies.

OBJECTIVE To investigate the odds of poor outcomes in emerging adulthood (ages 17 to 20 years) for children with different mental health trajectories at ages 9 to 13 years.

DESIGN, SETTING, AND PARTICIPANTS Growing Up in Ireland is a longitudinal, nationally representative population-based cohort study. Data collection began in August 2007 and was repeated most recently in September 2018. All results were weighted to account for sampling bias and attrition and were adjusted for socioeconomic factors. Data analysis took place from October 2022 to April 2023.

EXPOSURE Four latent classes captured variation in mental health in children aged 9 and 13 years, based on the parent-completed Strengths and Difficulties Questionnaire. Classes included no psychopathology, internalizing, externalizing, and high (comorbid) psychopathology. Those who remained in the same class from ages 9 to 13 years were included.

MAIN OUTCOMES AND MEASURES Poor functional outcomes in emerging adulthood were measured at approximate ages 17 years (range, 16 to 18 years) and 20 years (range, 19 to 21 years). Outcomes included poor mental health, poor physical health, social isolation, heavy substance use, frequent health service use, poor subjective well-being, and adverse educational/economic outcomes.

RESULTS Of 5141 included participants, 2618 (50.9%) were male. A total of 3726 (72.5%) were classed as having no childhood psychopathology, 1025 (19.9%) as having persistent externalizing psychopathology, 243 (4.7%) as having persistent internalizing psychopathology, and 147 (2.9%) as having persistent high psychopathology. Having any childhood psychopathology was associated with poorer functional outcomes in emerging adulthood. The internalizing group had elevated odds of most outcomes except for heavy substance use (range of odds ratios [ORs]: 1.38 [95% CI, 1.05-1.81] for frequent health service use to 3.08 [95% CI, 2.33-4.08] for poor mental health). The externalizing group had significantly elevated odds of all outcomes, albeit with relatively small effect sizes (range of ORs: 1.38 [95% CI, 1.19-1.60] for frequent health service use to 1.98 [95% CI, 1.67-2.35] for adverse educational/economic outcomes). The high psychopathology group had elevated odds of all outcomes (nonsignificantly for frequent health service use), though with wide confidence intervals (range of ORs: 1.53 [95% CI, 1.06-2.21] for poor physical health to 2.91 [95% CI, 2.05-4.12] for poor mental health). Female participants with any psychopathology had significantly higher odds of poor physical health and frequent health service use compared with male participants with any psychopathology.

CONCLUSIONS AND RELEVANCE In this longitudinal cohort study, childhood psychopathology was associated with a widespread pattern of functional impairment in emerging adulthood. Findings point to the need for a wider range of preventive interventions in child and adolescent mental health services.