During early adulthood, individuals from different cultures across the world tend to become more agreeable, more conscientious, and less neurotic. Two leading theories offer different explanations for these pervasive age trends: Five-factor theory proposes that personality maturation is largely determined by genetic factors, whereas social-investment theory proposes that personality maturation in early adulthood is largely the result of normative life transitions to adult roles. In the research reported here, we conducted the first systematic cross-cultural test of these theories using data from a large Internet-based sample of young adults from 62 nations ( N = 884,328). We found strong evidence for universal personality maturation from early to middle adulthood, yet there were significant cultural differences in age effects on personality traits. Consistent with social-investment theory, results showed that cultures with an earlier onset of adult-role responsibilities were marked by earlier personality maturation.