Section 311, Title 10, United States Code
The Militia
Militia: composition and classes
a) The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45
years of age and who are, or who have made a declaration of intention to become, citizens of the United States and female citizens of the United States
who are commissioned officers of the National Guard.
b) The classes of the militia are:
(1) the organized militia, which consists of the National Guard and the Naval Militia; and
(2) the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval Militia.