Background: Although plausible links between language development and personality have been suggested, longitudinal studies of these associations into adulthood have not been conducted. Aim: To investigate whether children's age at attaining language milestones is associated with later adult personality. Methods: Mothers' of 8,400 children from the Copenhagen Perinatal Cohort recorded six language milestones during the child's first 3 years of life. Information on at least one language milestone was available for two subsamples with adult follow-up information on personality: The Eysenck Personality Questionnaire was available for 691 individuals with a mean age of 27.5 years and The NEO Five-Factor Inventory was available for 902 individuals with a mean age of 50.0 years. Results: Faster attainment of language milestones was associated with lower neuroticism in young adulthood and with higher extraversion and openness to experience in midlife. A total of 1.9% of the variance in neuroticism, 2.5% of the variance in extraversion, and 1.6% of the variance in openness to experience was explained by language milestones. Conclusion: Only a modest part of variance in personality traits was explained by language milestones. However, the study adds to the literature on potential consequences of the timing of language development and suggests a link between language milestones and personality in adulthood.