poster boy vs adult male

poster boy and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
poster boy	Yes	No	No	No
adult male	Yes	No	No	No

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

Other hypernyms of poster boy include man, poster child.

poster boy (noun)	adult male (noun)
a male poster child	an adult person who is male (as opposed to a woman)