This study aimed to investigate the associations between early-and adult-life socioeconomic circumstances and physical inactivity (level and evolution) in aging using large-scale longitudinal data. This study used the Survey of Health Ageing and Retirement in Europe, a 10-yr population-based cohort study with repeated measurements in five waves, every 2 yr between 2004 and 2013. Self-reported physical inactivity (waves 1, 2, 4, and 5), household income (waves 1, 2, 4, and 5), educational attainment (wave of the first measurement occasion), and early-life socioeconomic circumstance (wave 3) were collected in 22,846 individuals 50 to 95 yr of age. Risk of physical inactivity was increased for women with the most disadvantaged early-life socioeconomic circumstances (odds ratio [OR], 1.49; 95% confidence interval [CI], 1.20-1.86). With aging, the risk of physical inactivity increased for both sexes and was strongest for those with the most disadvantaged early-life socioeconomic circumstances (OR, 1.04 (95% CI, 1.02-1.06) for women; OR, 1.02 (95% CI, 1.00-1.05) for men), with the former effect being more robust than the latter one. The association between early-life socioeconomic circumstances and physical inactivity was mediated by adult-life socioeconomic circumstances, with education being the strongest mediator. Early-life socioeconomic circumstances predicted high levels of physical inactivity at older ages, but this effect was mediated by socioeconomic indicators in adult life. This finding has implications for public health policies, which should continue to promote education to reduce physical inactivity in people at older ages and to ensure optimal healthy aging trajectories, especially among women with disadvantaged early-life socioeconomic circumstances.