poster girl vs adult female

poster girl and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
poster girl	Yes	No	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of poster girl; that is, adult female is a word with a broader meaning than poster girl:

Other hypernyms of poster girl include poster child, woman.

poster girl (noun)	adult female (noun)
a female poster child	an adult female person (as opposed to a man)