Flapper.
The word impertinent can mean rude, brash, or lacking respect. So, the phrase 'impertinent young girl' may mean a girl that is either brash or rude.
A "Flapper"
If you are looking for the crossword puzzle answer, a brash young woman could also be called a ladette.
ladette
1: A man that jumps to conclusions without researching the facts is considered brash.
She asks such brash questions.
Alan Brash was born in 1913.
Peter Brash was born in 1954.
Thomas Brash died in 1957.
Thomas Brash was born in 1874.
Brash Brands was created in 2006.
Brash Entertainment was created in 2007.