Smartphone Acquisition and Use at Age 13 Years and Health Outcomes at Age 14 Years

Importance: Smartphone acquisition and use under age 13 years is common and has been linked to adverse health implications. Although delaying acquisition until after age 13 years is often recommended, evidence supporting this guideline remains limited. Objective: To quantify the associations of smartphone acquisition around age 13 years and smartphone use with depression, obesity, and insufficient sleep at age 14 years. Design, Setting, and Participants: In this cohort study of youth from the Adolescent Brain Cognitive Development Study, eligible participants were study participants who did not have a smartphone by the 13-year-old assessment. Mixed-effects regression models were applied to quantify associations of past-year smartphone acquisition and use with health outcomes adjusting for pre-exposure depression, obesity and sleep, multiple socioeconomic and parental confounders, and use of other devices. Analyses were conducted between November 2025 and April 2026. Exposures: Smartphone acquisition and self-reported smartphone use duration (smartphone time). Main Outcomes: Odds ratios (ORs) for associations with depression diagnosis (based on a validated computerized diagnostic assessment), obesity (body mass index above 95th percentile), and insufficient sleep (less than 8 hours/night). Results: Participants included 1959 youth (803 girls [41.0%]; 292 Black [14.9%], 314 Hispanic [16.0%], 1661 White [84.8%]) followed up from the 3-year (mean [SD] age, 12.7 [0.6] years) to the 4-year (mean [SD] age, 14.0 [0.7] years) assessment. Between the 3- and 4-year assessments, 1230 participants acquired a smartphone and 729 did not. Smartphone acquisition was not significantly associated with depression (OR, 1.45; 95% CI, 0.98-2.14) or obesity (OR, 1.02; 95% CI, 0.71-1.46), but was associated with insufficient sleep (OR, 1.29; 95% CI, 1.03-1.62) at follow-up. Among those who had acquired a smartphone, total smartphone time (standardized z score) was associated with depression (OR, 1.22; 95% CI, 1.01-1.80), obesity (OR, 1.34; 95% CI, 1.09-1.65), and insufficient sleep (OR, 1.28; 95% CI, 1.12-1.47). Placing smartphones outside the bedroom at bedtime was associated with lower odds of insufficient sleep (OR, 0.64; 95% CI, 0.47-0.87). Conclusions and Relevance: The results of this cohort study suggest that simply smartphone acquisition at age 13 years was not associated with depression or with obesity, but was associated with insufficient sleep at age 14 years; however, the amount of smartphone use was associated with higher odds of all 3 outcomes. Behavioral interventions like limiting smartphone time and keeping smartphones out of bedrooms at night may protect adolescents from potential adverse health outcomes. Findings offer insights to guide caregivers on adolescent smartphone use and inform policies aimed at protecting youth, such as delaying smartphone acquisition until age 13 years.