sex kitten vs young woman

sex kitten and young woman both are nouns.

Word	Noun	Adjective	Verb	Adverb
sex kitten	Yes	No	No	No
young woman	Yes	No	No	No

As nouns, young woman is a hypernym of sex kitten; that is, young woman is a word with a broader meaning than sex kitten:

Other hypernyms of sex kitten include fille, girl, miss, missy, young lady.

sex kitten (noun)	young woman (noun)
a young woman who is thought to have sex appeal	a young female