3. You're really stretching.

"Anyone ever heard a phrase used like, 'the boys in the Army'?"

Not since about World War II. How would you feel about sending "boys" to be slaughtered?

And in today's volunteer army / job corps, I'm presuming they are hardly boys, but (warning: cynicism alert) humans alert enough and with enough agency to have chosen the armed services as the best route to whatever they think their future holds.

I've not heard all the troops in Afghanistan, Iran, etc., over the years referred to as "boys."

>> Both "boys" and "girls" are commonly used to indicate gender, not necessarily age.

Really? Call a 40-year-old woman a girl these days and see what happens. (And please don't use your very useful wife as a one-person cohort.)