Title 10, U.S. Code, Section 311 states that the militia consists of all able-bodied males aged from 17 to 45, both citizens and those who have declared their intent to become citizens, and female citizens who are officers of the National Guard. It also specifies that the militia consists of two classes: the organized militia (the National Guard and the Naval Militia) and all of the rest, the unorganized militia or reserve militia.
Title 10, U.S. Code, Section 312 lists persons exempted from militia service as the vice president, governors, judges, members of the armed forces, pilots of navigable waters, merchant mariners, custom house clerks, and postal and arsenal employees.
Title 32 U.S. Code, Section 109 authorizes states in peacetime to establish state defense forces in addition to their National Guard units
Title 32, U.S. Code, Section 313 adds as members of the militia those persons under age of 64 who are former members of the regular armed forces of the nation