Subjective Well-Being is related to the Big-Five and to Individualistic and Collectivistic beliefs of Polish adolescents. In the present study, we examined whether Individualism and Collectivism beliefs mediate between the Big-Five and Subjective Well-being among adolescents, young and middle-aged adults. Adolescents (N = 174, 36% men, aged 14–18), young (N = 254, 45% men, aged 19–24) and middle-aged adults (N = 252, 54% men, aged 40–55) completed the NEO-FFI, the Ind-Col20, and measures of Subjective Well-being. The three groups differed on all dimensions. Adolescents reported the highest Neuroticism, the lowest Agreeableness, Conscientiousness, the highest Individualism and Collectivism beliefs and lowest SWB. Among adolescents, SEM analyses indicated that Subjective Well-being was negatively related to Neuroticism and Agreeableness, positively to Extraversion, Conscientiousness, Horizontal Individualism, Horizontal and Vertical Collectivism. Among young and middle-aged adults Subjective Well-being was negatively related to Neuroticism and Horizontal Collectivism, positively to Openness, Conscientiousness, Horizontal and Vertical Individualism. Beliefs partially mediated the effects of traits. Relationships were different for cognitive and affective Subjective Well-being indices.