party girl vs young lady

party girl and young lady both are nouns.

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

As nouns, young lady is a hypernym of party girl; that is, young lady is a word with a broader meaning than party girl:

party girl: an attractive young woman hired to attend parties and entertain men
young lady: a young female

Other hypernyms of party girl include fille, girl, miss, missy, young woman.

party girl (noun)	young lady (noun)
an attractive young woman hired to attend parties and entertain men	a young female