Significance This comprehensive life-course longitudinal study of body mass index (BMI) contributes knowledge about the temporal dynamics of BMI as individuals age and how social status defines disparities in lifetime obesity risk. Results show that adolescence and young adulthood are critical life stages when excess weight can rapidly accumulate and racial/ethnic or educational disparities emerge, most significantly among recent cohorts of young people. These cohort increases in the prevalence and rate of increase in obesity have alarming consequences for contemporary epidemiologic conditions such as the COVID-19 pandemic and US population health and life expectancy in years to come. Our study further contributes data analytic tools by linking and integrating multiple existing studies to advance science on social determinants of health.