girl vs young lady

girl and young lady both are nouns.

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

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

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

girl (noun)	young lady (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