Subjective age predicts consequential outcomes in old age, including risk of hospitalization, dementia, and mortality. Studies investigating the determinants of subjective age have mostly focused on aging-related factors measured in adulthood and old age. Little is known about the extent to which early life factors may contribute to later life subjective age. The present study examined the prospective association between IQ in adolescence and subjective age in later life and tested education, disease burden, adult cognition, and personality traits as potential mediators. Participants (N = 4494) were drawn from the Wisconsin Longitudinal Study. Data on IQ were obtained in 1957 when participants were in high school. Education, disease burden, cognition, and personality were assessed in 1992–1993, and subjective age was measured in 2011 at age 71 (SD = 0.93). Accounting for demographic factors, results revealed that higher IQ in adolescence was associated with a younger subjective age in late life. Bootstrap analysis further showed that this association was mediated by higher openness. The present study suggests that how old or young individuals feel is partly influenced by lifespan developmental processes that may begin with early life cognitive ability.