Section 311 of US Code Title 10:
(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 who are, or who have made a
declaration of intention to become, citizens of the United States
and of female citizens of the United States who are members 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.
In memoriam, members of the unorganized militia of Flight 93.