General Discussion

Using the words "girl" and "woman" as you describe has some ambiguity because your usage is not the only way those words are used... and it's not a sexist issue because the same situation exists with "boy" and "man."

A 20 year old woman might tell a parent/friend, "I met a really nice boy last night." A 20 year old man might tell a parent/friend, "I met a really nice girl last night." In neither case is the parent/friend likely to interpret that as meaning they met a nice underage potential romantic companion. And these people they met may turn into, you guessed it, boyfriends or girlfriends.

Similarly, an under-18 person may sometimes be called a "nice young man" or a "nice young woman," particularly by people of an older generation. You may prefer they didn't, but it is not a terribly uncommon usage. I"m not saying that your definitions are wrong, I'm just saying that that is not the only way the words are used IRL.

As mentioned by others, "minor" and "underage female" would be some possible unambiguous options.

I do see your point that referring to Epstein's victims as young women is problematic. But your headline and first two sentences put forth that you think they should simply be called "girls" (since "underage" would be redundant). But because of common usage, that is inadequate. "Epstein invited powerful people to his island to have sex with girls he supplied" does not make clear that these females are minors. In this case, your proposed fix of calling the victims girls rather than underage women can actually make the activity seem LESS nefarious. Because whether you like it or not, 18+ year old females are often referred to as girls, in common conversation, and in decades of songs and movies. Heck, prostitutes are "call girls," not "call women." So calling Epstein's victims simply "girls" does them a disservice, as it is LESS clear to the reader/listener that they are underage than if they were referred to as "underage women."

It might be best to simply say they were having sex with minors. There's enough horror right there, regardless of the gender of the victims. But if you want to make clear that the victims are female, there has to be something that makes clear to the general public that they were underage, and the word "girl" simply doesn't do that, because the word is so widely used in other ways. People may hear the word "girls" and not assume they were necessarily under the age of consent, because your usage of the word is not universal, and is not the way everyone will automatically interpret the word. It is important to communicate to all that these people were underage, and I believe that simply calling them 'girls" does not do that.