Epigenetics is the study of how people’s behavior and environments influence the way their genes are expressed, even though their DNA sequence is itself unchanged. By aggregating age-related epigenetic markers, epigenetic ‘clocks’ have become the leading tool for studying biological aging. We make an important contribution by developing a novel, integrated measure of epigenetic aging – the Multi EpiGenetic Age (MEGA) clock – which combines several existing epigenetic clocks to reduce measurement error and improve estimation efficiency. We use the MEGA clock in three empirical contexts to show that: i) accelerated epigenetic aging in adolescence is associated with worse educational, mental-health, and labor market outcomes in early adulthood; ii) exposure to child maltreatment before adolescence is associated with half a year higher epigenetic aging; and iii) that entering school one year later accelerates epigenetic aging by age seven, particularly among disadvantaged children. The MEGA clock is robust to alternative methods for constructing it, providing a flexible and interpretable approach for incorporating epigenetic data into a wide variety of settings.