The DU Lounge

It's a holdover from when the word "woman" referred to middle-aged and elderly women. Young women were offended to be called women, and preferred to be called girls (after "gals" became outdated). Some younger women still hold on to this idea, even though there's nothing at all offensive about being called a woman.

I have found that the best compromise is to call adult "girls" young women. Adding the word "young" makes it clear that you aren't talking about older women, so nobody gets offended.