Framed using affection exchange theory, this study aimed to provide a better understanding of grandparent-grandchild relationships during adolescence and emerging adulthood by testing the mediating role of affectionate communication between the grandchild's perception of grandparent-grandchild personality trait similarities and grandchildren's emotional closeness with their grandparents. 710 French adolescents and emerging adults completed surveys in the context of their relationship with a specific grandparent. Results indicated that adolescents perceived more similarity in their agreeableness, conscientiousness, and neuroticism with their grandparents compared to emerging adults, but no significant difference was found for received grandparental affectionate communication. Adolescents also reported greater emotional closeness with their grandparents compared to emerging adults. Mediation analyses partially supported the hypothesized role of received grandparental affection between personality traits similarities and emotional closeness. These findings highlight the role of similarity in personality traits and the importance of affectionate communication to enhance emotional closeness.