A disability pension shall not be paid unless there is filed with the board certificates of the police officer’s disability, subscribed and sworn to by the police officer if not under legal disability, or by a representative if the officer is under legal disability, and by the police surgeon (if there be one) and 3 practicing physicians selected by the board. The board may require other evidence of disability. Medical examination of a police officer retired for disability shall be made at least once each year prior to attainment of age 50, as verification of the continuance of disability for service as a police officer. No examination shall be required after age 50.