The commissioned and noncommissioned officers throughout all of the departments, corps, regiments and lesser organizations shall conform to those of similar organizations in the army of the United States: Provided, however, That the minimum age for commissioning of officers is reduced to the age of eighteen (18) for either graduates of accredited officer candidate schools, or commissioned officers of the army reserve who apply for federal recognition in the army national guard. Whenever vacancies occur in any of the commissioned grades they shall be filled in accordance with rules and regulations prescribed by the military board and approved by the commander in chief.