You know, we do have a militia, and we don't do anything with it

[petronius]

4. It probably varies by state, but in CA the definition is

The militia of the State consists of all able-bodied male
citizens and all other able-bodied males who have declared their
intention to become citizens of the United States, who are between
the ages of eighteen and forty-five, and who are residents of the
State, and of such other persons as may upon their own application be
enlisted or commissioned therein pursuant to the provisions of this
division, subject, however, to such exemptions as now exist or may be
hereafter created by the laws of the United States or of this State.

So it sounds to me like men 18-45 are automatically counted (with some exemptions), but anyone else outside that category can be included at their own discretion...