No enlistment shall be allowed other than of able-bodied citizens of the United States and able-bodied persons of foreign birth who have been admitted to the United States for permanent residence. Such citizens or persons are required to be between the ages of seventeen and sixty years. All enlistments, reenlistments, and extensions of enlistments in the National Guard shall be in the manner and form, and for such periods as may be authorized by the laws of the United States, and regulations of the Secretary of the Army and the Secretary of the Air Force relating thereto, and shall be made by signing enlistment papers according to the form prescribed by the Adjutant General, and taking the prescribed oath or affirmation, which may be administered by any commissioned officer.