Woman or Girl?

Rosa and Kate are two main characters in Lives Interrupted.  I didn't specify their ages in the book as their lives and problems should give enough clues, and I like to leave things up to the reader.  In my character outline Kate is early twenties, and Rosa in her early thirties.  Kate is single, while Rosa is married with a young son.

I struggled for a while over whether to call Kate a girl, or a woman.  I didn’t have the same problem with Rosa, as I felt she was definitely not a girl. Was this because of her age, or that she was married and a mother.  Rhetorical question really as I don't know, it was just how I felt about these two particular characters.  Obviously someone of Kate's age, or younger, can be married and a mother.  Would this change my conundrum of her being a girl or a woman?

I was reminded of this through a Twitter conversation I had earlier with a writer friend, and I asked her at what age she thought someone was no longer a girl.  She said eighteen, which sounds reasonable. What are your thoughts?