nobleman vs adult male

nobleman and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
nobleman	Yes	No	No	No
adult male	Yes	No	No	No

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

Other hypernyms of nobleman include male aristocrat, man.

nobleman (noun)	adult male (noun)
a titled peer of the realm	an adult person who is male (as opposed to a woman)