Well someone who is 18 is also a “young woman.” So someone who is 17 would be an “underage young woman.” But that’s clunky – thus, “underage woman.” IDK why people think this is a psy-op, seems like natural English to me.

I was called “young woman” when I was 15, maybe earlier. Is this abnormal?

I don’t think that your definition of the term “sex” is universal.