Importance The relationship of neighborhood environmental and socioeconomic factors with mental health across childhood and adolescence remains unclear.

Objective To investigate the associations of neighborhood characteristics with mental health at various developmental stages, from early childhood to late adolescence.

Design, Setting, and Participants This population-based cohort study used data from the Millennium Cohort Study, including approximately 19 000 children born in the UK from 2000 to 2002. This analysis included individuals in England with complete data on neighborhood exposures at ages 3, 5, 7, 11, 14, and 17 years. Data were analyzed from January to December 2023.

Exposures Neighborhood-level air pollution, green space, and socioeconomic status.

Main Outcomes and Measures Strengths and Difficulties Questionnaire (SDQ) scores linked with exposure to air pollution, green space, and socioeconomic status, measured at participants’ residential addresses, were analyzed using a hierarchical bayesian regression model. Hypotheses were formulated after data collection.

Results The sample included 3595 children and adolescents, with 1826 (50.5%) female; 3012 participants (83.8%) were White and 583 participants (16.2%) were another ethnicity. The mean (SD) SDQ score was 7.1 (5.1). After adjusting for individual and household factors, neighborhood socioeconomic status emerged as the strongest factor associated with mental health. Residing in affluent neighborhoods was associated with improved mental health, with this association magnifying through adolescence. For children of the same age, those in the most affluent neighborhoods had log-transformed SDQ scores 0.73 (95% credible interval [CrI], 0.58 to 0.88) lower at age 17 years compared with their peers in the most deprived areas; at age 5 years, scores were 0.31 (95% CrI, 0.17 to 0.45) lower. Exposure to particulate matter with diameter less than 2.5 μm was associated with poorer mental health during early childhood (age 3 years), with an effect size of 0.15 (95% CrI, 0.08 to 0.22). While green space was not directly associated with improved mental health, a sex-based difference was observed, with males showing more favorable associations (interaction, −0.10 [95% CrI, −0.17 to −0.03]).

Conclusions and Relevance In this cohort study of children and adolescents, associations between neighborhood characteristics and mental health evolved from childhood through adolescence. These findings suggest that targeted interventions in disadvantaged neighborhoods and strategies to protect young children from air pollution are essential. A comprehensive approach is recommended to incorporate air pollution, green space, and socioeconomic status not only in residential neighborhoods but also in other settings, such as schools.