IMPORTANCE: Although a patient's age may be the only objective figure that can be used as a reference indicator in selecting the type of prosthesis in heart valve surgery, different clinical guidelines use different age criteria. OBJECTIVE: To explore the age-associated survival-hazard functions associated with prosthesis type in aortic valve replacement (AVR) and mitral valve replacement (MVR). DESIGN, SETTING, AND PARTICIPANTS: This cohort study compared the long-term outcomes associated with mechanical and biologic prostheses in AVR and MVR according to recipient's age using a nationwide administrative data from the Korean National Health Insurance Service. To reduce the potential treatment-selection bias between mechanical and biologic prostheses, the inverse-probability-of-treatment-weighting method was used. Participants included patients who underwent AVR or MVR in Korea between 2003 and 2018. Statistical analysis was performed between March 2022 and March 2023. EXPOSURES: AVR, MVR, or both AVR and MVR with mechanical or biologic prosthesis. MAIN OUTCOMES AND MEASURES: The primary end point was all-cause mortality after receiving prosthetic valves. The secondary end points were the valve-related events, including the incidence of reoperation, systemic thromboembolism, and major bleeding. RESULTS: Of the total of 24 347 patients (mean [SD] age, 62.5 [7.3] years; 11 947 [49.1%] men) included in this study, 11 993 received AVR, 8911 received MVR, and 3470 received both AVR and MVR simultaneously. Following AVR, bioprosthesis was associated with significantly greater risks of mortality than mechanical prosthesis in patients younger than 55 years (adjusted hazard ratio [aHR], 2.18; 95% CI, 1.32-3.63; P = .002) and in those aged 55 to 64 years (aHR, 1.29; 95% CI, 1.02-1.63; P = .04), but the risk of mortality reversed in patients aged 65 years or older (aHR, 0.77; 95% CI, 0.66-0.90; P = .001). For MVR, the risk of mortality was also greater with bioprosthesis in patients aged 55 to 69 years (aHR, 1.22; 95% CI, 1.04-1.44; P = .02), but there was no difference for patients aged 70 years or older (aHR, 1.06; 95% CI, 0.79-1.42; P = .69). The risk of reoperation was consistently higher with bioprosthesis, regardless of valve position, in all age strata (eg, MVR among patients aged 55-69 years: aHR, 7.75; 95% CI, 5.14-11.69; P < .001); however, the risks of thromboembolism and bleeding were higher in patients aged 65 years and older after mechanical AVR (thromboembolism: aHR, 0.55; 95% CI, 0.41-0.73; P < .001; bleeding: aHR, 0.39; 95% CI, 0.25-0.60; P < .001), with no differences after MVR in any age strata. CONCLUSIONS AND RELEVANCE: In this nationwide cohort study, the long-term survival benefit associated with mechanical prosthesis vs bioprosthesis persisted until age 65 years in AVR and age 70 years in MVR.