young buck vs adult male

young buck and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
young buck	Yes	No	No	No
adult male	Yes	No	No	No

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

Other hypernyms of young buck include adolescent, stripling, teen, teenager, man.

young buck (noun)	adult male (noun)
a teenager or a young adult male	an adult person who is male (as opposed to a woman)