Using the 1957–2011 data from 10,317 participants in the Wisconsin Longitudinal Study, I examine how socioeconomic status (SES) at age 18 affects all-cause mortality between ages 18 and 72. Integrating fundamental cause theory, gender relations theory, and a life course perspective, I evaluate the cumulative advantage (CA) and age-as-leveler processes as well as gender differences in these processes. Findings indicate that higher early-life SES at age 18 is related to lower mortality over the life course, and the effect of early-life SES is not explained by socioeconomic achievement and health behaviors in adulthood. Consistent with the CA model, early-life SES generates increasing within-cohort inequality with age, and this CA process is stronger for women than men. Results also show that unequal selection by SES obscures the CA process and creates an illusion of the age-as-leveler process. This study calls for a lifelong gendered approach to socioeconomic health disparities.