OBJECTIVE: Lifespan perspectives on personality development have gained much momentum in recent years, mostly focusing on benevolent and neutral traits such as the Big Five. Despite their strong associations with critical personal outcomes, surprisingly little research has investigated the development of malevolent traits. Addressing this gap, we examined age trends in Machiavellianism across the lifespan. METHODS: Using data from a large-scale cross-sectional sample (n = 1,118,643), we analyzed mean-level changes from age 10 to 67. RESULTS: Age differences in Machiavellianism were most pronounced as a strong upward trend during the transition from late childhood to adolescence, when it peaked. Throughout adulthood it exhibited a steady downward trend, reaching an overall minimum at age 65. Across the lifespan, Machiavellianism tended to be higher in men and high-income participants. Compared to Machiavellianism, the age trends in Agreeableness and-to a lesser extent-Conscientiousness showed almost perfectly polar opposite patterns. CONCLUSIONS: Age trends in malevolent personality conform to established patterns of normative change, indicating temporary disruption in adolescence and social maturation across adulthood. The results advance theory and research on personality trait development across the lifespan and highlight crucial developmental windows that can inform targeted interventions to keep socially aversive traits in check.