young man vs young buck

young man and young buck both are nouns.

Word	Noun	Adjective	Verb	Adverb
young man	Yes	No	No	No
young buck	Yes	No	No	No

As nouns, young man and young buck are synonyms defined as:

young man (noun)	young buck (noun)
a teenager or a young adult male	a teenager or a young adult male