You probably already know this, but I wanted to insert the section of the US Code that backs you up:
TITLE 10, Subtitle A, PART I, CHAPTER 13, Section 311. 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 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.
Pretty much means just about everyone in the USA is either part of the militia or “retired” from it. If the person has prior military service, the age is extended to 64. With retirement ages increasing, and in time of need, I am sure the upper age range would be waiverable.
Title 10 should be required reading (study) in every high school in the U.S.A.
Where did you find that exception?