Against the backdrop of the increasingly severe aging situation, exploring the implementation mechanism of knowledge sharing behaviors among older employees holds significant practical significance for fully developing the human resources of the elderly and promoting sustainable corporate development. Many studies have shown that responsibility orientation is one of the most important factors influencing older employees'' proactive behaviors that benefit the organization. However, little attention has been paid to the impact of responsibility orientation on knowledge sharing among older employees. In light of this, to understand how and when responsibility orientation influences knowledge sharing among older employees, this study constructs a moderated mediation model based on the proactive motivation model, investigating the mechanisms and boundary conditions through which responsibility orientation affects older employees'' knowledge sharing behaviors. First, responsibility orientation has a positive influence on knowledge sharing among older employees through their sense of responsibility towards members, the mission, and norms (Hypothesis 1). Second, responsibility orientation positively influences knowledge sharing behaviors among older employees by reinforcing their sense of obligation towards organizational development (Hypothesis 2). Finally, the perception of an anti-ageism climate strengthens the favorable effects of responsibility orientation by creating a fair and inclusive environment for older employees (Hypotheses 3 and 4). Linear regression and Monte Carlo bootstrapping were employed to analyze data from 200 paired samples of employees aged 40 and above across various industries such as manufacturing, finance, and education. A week before the daily surveys began, participants received a questionnaire containing demographic information (e.g., survey code, age, gender, education, industry, tenure, and tenure with the current organization). Data for this research were collected at three time points. At time 1, employees completed measurements on demographics, responsibility orientation, and constructive responsibility perception. At time 2, employees completed measurements on the perception of an anti-ageism climate. At time 3, employees completed measurements on knowledge sharing behaviors. Path analysis using Mplus 8 was conducted to test the hypotheses, and Monte Carlo bootstrapping using R 3.5.1 was employed to further verify the mediating and moderated mediation effects of the proposed model. The results showed that (1) duty orientation positively influences knowledge sharing among older employees by enhancing their constructive responsibility perception; (2) the perception of an anti-ageism climate significantly moderates the relationship between duty orientation and constructive responsibility perception, with a stronger positive effect of duty orientation when the perception of an anti-ageism climate is high, while the relationship is not significant when the perception of an anti-ageism climate is low; and (3) the perception of an anti-ageism climate positively moderates the mediating effect of constructive responsibility perception on the relationship between responsibility orientation and knowledge sharing behaviors. This study has several theoretical implications: Firstly, by focusing on duty orientation as an important personal characteristic of older employees and investigating its influence on knowledge sharing behaviors, this study enriches the research on antecedents of knowledge sharing behaviors among older employees. Secondly, by using the proactive motivation model and selecting constructive responsibility perception as a mediating variable, this study expands the research on the mediating role of responsibility orientation in the relationship with knowledge sharing behaviors. Lastly, based on the proactive motivation model, this study explores the interaction between employee duty orientation and the organizational anti-ageism climate in influencing constructive responsibility perception, elucidating how different factors, including individual and organizational contexts, interact to affect individuals'' proactive motivation and knowledge sharing behaviors. This research enriches the boundary conditions of the influence of duty orientation and provides a more comprehensive answer to how organizations can stimulate knowledge sharing behaviors among older employees.