The militia of the Commonwealth of Virginia shall consist of all able-bodied citizens of this Commonwealth and all other able-bodied persons resident in this Commonwealth who have declared their intention to become citizens of the United States, who are at least sixteen years of age and, except as hereinafter provided, not more than fifty-five years of age. The militia shall be divided into four classes, the National Guard, which includes the Army National Guard and the Air National Guard, the Virginia State Defense Force, the naval militia, and the unorganized militia.