In the "rough music" scene towards the beginning of I Shall Wear Midnight, when Tiffany and her father are taking care of Amber, who was beaten by her father hard enough that she miscarried a baby, Tiffany insists on using this phrase "young woman" for Amber:
Mr Aching stood up. 'I think we should take this girl home, don't you?'
'Young woman,' corrected Tiffany, leaning over her.
"Young woman,' said Tiffany. 'She deserves that, at least. And I think I should take her somewhere else first. She needs more help than I can give her. Can you please go and scrounge some rope? I've got a leather strap on the broomstick, of course, but I don't think it will be enough.' She heard a rustling from the hayloft above, and smiled. Some friends could be so reliable.
I Shall Wear Midnight, chapter 2: "Rough Music"
Yet after that one scene, everyone reverts to using "girl", including Tiffany and the narrator:
Tiffany yawned again. 'Thank you for offering, Jeannie,' she said, 'but I don't think I need them, if it's all the same to you.' There was a pile of greasy fleeces in the corner that had probably not long before belonged to sheep who had decided to say goodbye to the cruel world and commit suicide. They looked very inviting. 'I had better go and see to the girl.' Tiffany's legs did not seem to want her to move. 'Still, I expect she is as safe as houses in a Feegle mound.'
'Oh, no,' said Jeannie softly as Tiffany's eyes shut. 'Much, much safer than houses.'
I Shall Wear Midnight, chapter 3: "Those Who Stir In Their Sleep"
Tiffany's father pulled his daughter to one side and lowered his voice. 'Petty came back in the night,' he hissed, 'and they say that someone tried to kill him!'
'True as I'm standing here.'
Tiffany turned to Amber. The girl was staring at the sky as if hoping patiently for something interesting to happen.
I Shall Wear Midnight, chapter 4: "The Real Shilling"
Why, then, does Tiffany insist on the phrase "young woman" in chapter 2? It seems to me that using "girl" would help drive home the brutality of Petty's act more than "young woman"; he beat a 13-year-old girl. 13 is young enough that "girl" seems apt. What does insisting on "young woman" accomplish when not even the narrator uses that phrase after that one scene?