Key PointsQuestionWhich type of prosthesis is associated with the best outcomes by age of the recipient undergoing aortic or mitral valve replacement?FindingsThis cohort study of 24 347 patients who underwent aortic or mitral valve replacement compared the long-term outcomes associated with mechanical and bioprostheses according to the recipient’s age. The mechanical prosthesis was associated with a survival benefit over bioprosthesis, and the benefit was maintained in patients up to age 65 years for replacements in the aortic position and age 70 years for replacements in the mitral position.MeaningThe findings of this study may encourage health care practitioners to adopt a more conservative approach in choosing a prosthesis type and provide information for better shared decision-making for both patients and physicians.