The board may require any disability beneficiary under age 55 to undergo medical examination. The examination shall be made by a physician or surgeon appointed by the board at the place of residence of the beneficiary or other place mutually agreed upon. Upon the basis of the examination the board shall determine whether the disability beneficiary is still physically or mentally incapacitated for service in the office or department of the county or district where he was employed and in the position held by him when retired for disability.