The age should be 17 to buy or own any firearm.
https://www.law.cornell.edu/uscode/text/10/246#:~:text=The%20militia%20of%20the%20United%20States%20consists%20of,States%20who%20are%20members%20of%20the%20National%20Guard.
Alas, the challenge of your cite of title 10 would be fine except that as of age 45 you would no longer be part of the militia. Not sure that's goin to work for me.
Although, as a member of the Inactive Naval Reserve ready reserve for another year and half before I transfer to the retired rolls, I suspect I fall under class #1
(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.(Aug. 10, 1956, ch. 1041, 70A Stat. 14, § 311; Pub. L. 85–861, § 1(7), Sept. 2, 1958, 72 Stat. 1439; Pub. L. 103–160, div. A, title V, § 524(a), Nov. 30, 1993, 107 Stat. 1656; renumbered § 246, Pub. L. 114–328, div. A, title XII, § 1241(a)(2), Dec. 23, 2016, 130 Stat. 2[/i]497[/i].)
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