Officers of the national guard shall be selected from the classes of persons having the qualifications prescribed by federal law and regulations. They shall be appointed by the governor upon the recommendation of their superiors in the chain of command, provided that they shall have successfully passed such tests as to physical, moral, and professional fitness, as shall be prescribed by law and regulations. Each officer shall take an oath of office and shall hold office until the officer shall have attained the maximum age of retirement that is prescribed by federal law or regulations pertaining to officers of the armed forces of the United States, unless the officer’s commission or warrant is sooner vacated by resignation, death, or as provided in this chapter. In case the officer has no immediate superiors, within the state, in the chain of command, the officer shall be appointed, as provided in this section, upon the recommendation of the adjutant general. A commission shall designate the arm or branch of service in which the officer is commissioned. Provided, however, that no person shall be appointed a commissioned or warrant officer who has not reached the person’s eighteenth birthday at or prior to the time of such appointment.