It's not in the Constitution, it's in the U.S. Code.
Trying to put my hands on the actual citation, I looked through the REPORT of the SUBCOMMITTEE ON THE CONSTITUTION of the UNITED STATES SENATE, NINETY-SEVENTH CONGRESS, Second Session, February 1982:

| In the Militia Act of 1792, the second Congress defined "militia of the United States" to include almost every free adult male in the United States. These persons were obligated by law to possess a firearm and a minimum supply of ammunition and military equipment. Act of May 8, 1792; Second Cong., First Session, ch. 33. This statute, incidentally, remained in effect into the early years of the present century as a legal requirement of gun ownership for most of the population of the United States |
Not finding the current law there, I looked in the MEMORANDUM OPINION FOR THE ATTORNEY GENERAL, "WHETHER THE SECOND AMENDMENT SECURES AN INDIVIDUAL RIGHT," dated August 24, 2004:

| This common sense of “Militia” also appeared in the House of Representatives’ debates on the Second Amendment, discussed below in Part III.C.2, and the Second Congress applied it in the first Militia Act, enacted in 1792, two months after the Second Amendment was officially ratified. The Act required “each and every able-bodied white male citizen of the respective states, resident therein, who is or shall be of the age of eighteen years, and under the age of forty-five years,” to be “enrolled in the militia” by the local commanding officer. Each enrolled citizen was required to provide his own arms – “a good musket or firelock” or “a good rifle” – plus ammunition and accouterments. These private arms were exempted from “all suits, distresses, executions or sales, for debt or for the payment of taxes.” The enrollees were required to appear, armed, “when called out to exercise, or into service,” although Congress left the details of exercise to each State.102 (Since 1792, Congress has only expanded this definition, such as by eliminating the racial restriction and including some women.103) Finally, Noah Webster in his 1828 American dictionary defined “militia” in accord with this Act and the above understanding: “The militia of a country are the able bodied men organized into companies, regiments and brigades, with officers of all grades, and required by law to attend military exercises on certain days only, but at other times left to pursue their usual occupations.” They were “enrolled for discipline, but not engaged in actual service except in emergencies.”104 |
Footnote 103 cites: 10 U.S.C. § 311(a):

(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.
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