Objectives: Personality is known to be a reliable predictor of well-being. However, it is rather difficult to influence the personality of individuals in order to improve their well-being. Therefore, it is important to examine possible underlying mechanisms or indirect effects. Consequently, the aim of the current study was to investigate whether psychological flexibility is a mechanism explaining the relationship between personality and well-being. Given the evidence that age-related differences exist in personality, flexibility, and well-being, we also investigated whether our indirect effects model differed in both older and younger adults.Design: We used a cross-sectional design.Setting: Participants were asked to fill in questionnaires at home.Participants: We recruited 138 younger (25-50 years) and 120 older (65+) adults from a community-dwelling population.Measurements: Self-report questionnaires were used to assess (mal)adaptive personality traits (Big Five), psychological flexibility, and affective and general subjective well-being.Results: Similar indirect effects were found in older and younger adults: Psychological flexibility is a mechanism explaining the link between personality and well-being. In nearly half of the models, psychological flexibility even fully accounted for the effect of personality on well-being.Conclusion: These results have important implications for clinical practice, since psychological flexibility, contrary to personality traits, is malleable. Interventions to increase psychological flexibility already exist and are validated in both older and younger samples. They may hold promise to improve well-being.