Personality plays a role in phenotypic risk and resilience across the lifetime. Research on personality development during adolescence is mixed and suggests both increases and decreases in individual traits (e.g. extroversion, agreeableness, etc.) with age. To address these disparate findings, this study capitalized on a relatively large study, with multiple waves of longitudinal data, to investigate linear and non-linear changes in personality across adolescence and young adulthood and as a function of sex. This study replicated previous reports of increasing conscientiousness and agreeableness with age, and provides novel evidence of non-linear changes in extroversion and emotional stability as a function of sex. These sex differences and non-linear patterns in personality development help reconcile conflicting reports of personality change. Future research should attempt to replicate these sex differences in development on more comprehensive measures of personality.