Edit: I had always heard the phrase "separate the men from the boys" in that form rather than the feminine form. And I think there's a reason there's a differ

Edit: I had always heard the phrase "separate the men from the boys" in that form rather than the feminine form. And I think there's a reason there's a different meaning, that's pretty intricately tied to social notions of gender. While gender is largely socially constructed, rites of passage for respective genders are even more so. In many cultures, young men go through a rite of passage to prove they're men. They prove their masculinity--not show it, prove it, usually overcoming some obstacle. They kill a lion, for instance. Women go through rites of passage too, but they do not involve proof, rather show. This does not mean the challenges involved are easier. It simply means their role in the final evaluation is passive rather than active. The reason men are active in their judgment and women passive reflects back on the opposite gender, because men judge women and women judge men. Men are looking for traits that are visible, women for traits of character. You could argue this is all because men are shallow and women are not, and you'd be right for reasons due, in turn, with the gametic, game-theoretical origin of asymmetries between men and women. So there are fundamental reasons why it's not the same thing to say "separate the men from the boys" and "separate the women from the girls."