Based on the perspective of person-supervisor fit, this study examines how leader-follower optimism fit affects proactive customer service performance among frontline service employees through cognition-based trust and affective commitment, as well as the moderating role of leader-follower age fit in this relationship. By analyzing three-wave matched survey data from 213 dyads of leaders and followers, the results of polynomial regression and response surface analysis show that leader-follower optimism fit promotes proactive customer service performance among frontline service employees. In case of optimism fit, proactive customer service performance is higher when the leader and follower are both highly optimistic than when they are both low in optimism. Cognition-based trust and affective commitment play a mediating role between leader-follower optimism fit and proactive customer service performance. Besides, age fit moderates these relationships. Compared with cases in which the leader is younger than or the same age as the follower, when the leader is older, the mediating role of cognition-based trust is stronger; compared with cases involving a large age gap between the leader and the follower, the mediating role of affective commitment is stronger.