OBJECTIVES: To test whether early-life factors (education, height, father's social position) and midlife social, behavioral, and psychosocial factors were associated with entering older age without disease and with good functioning. DESIGN: A longitudinal, British civil service-based cohort study. Participants were followed for 17 years to assess successful aging. This was defined as being free of major disease and in the top tertile of physical and cognitive functioning measured in 2002 to 2004. SETTING: Twenty London-based Civil Service departments. PARTICIPANTS: Four thousand, one hundred forty men and 1,823 women, free of major disease at baseline in 1985 to 1988 (mean age 44, range 35-55). MEASUREMENTS: Behavioral, biological, and psychosocial risk factors; physical and cognitive functioning; and disease outcomes. RESULTS: Five hundred forty eight (12.8%) men and 246 (14.6%) women were successfully aging at follow-up. Midlife socioeconomic position strongly predicted this (age-adjusted odds ratio, highest vs lowest=7.1, 95% CI=3.4-14.6, for men and 7.7, 95% CI=4.9-12.1, for women). Height, education (in men), not smoking, diet, exercise, moderate alcohol (in women), and work support (in men) were related to a favorable older life after adjustment for age and socioeconomic position. CONCLUSION: Interventions to promote healthy adult behavior may attenuate harmful effects of less-modifiable risk factors and reduce social inequalities.