young woman vs girl

young woman and girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
young woman	Yes	No	No	No
girl	Yes	No	No	No

As nouns, young woman and girl are synonyms defined as:

Other synonyms of young woman include fille, miss, missy, young lady.

young woman (noun)	girl (noun)
a young female	a young female
	a girl or young woman with whom a man is romantically involved
	a youthful female person
	a friendly informal reference to a grown woman
	a female human offspring