Any person under the age of seventy-six who receives retirement compensation under the provisions of sections 476.515 to 476.570 may engage in the practice of law or do law business at any time after his retirement if he makes himself available to serve as appointed defense counsel for indigent persons who may be charged with a violation of any of the criminal laws of this state. Such appointments shall not exceed six cases involving prosecution for a violation of a criminal law classified as a felony in any calendar year, and any person so appointed shall not receive any compensation therefor but may be reimbursed for his actual and necessary out-of-pocket expenses from funds appropriated for the support of public defender activities. The supreme court may excuse any such person from so serving as appointed defense counsel upon a showing of physical or mental condition which would preclude such service. Any person of the age of seventy-six or over who retires or is retired under the provisions of sections 476.515 to 476.570 may engage in the practice of law without making himself available to serve as appointed defense counsel.