teenager vs young buck

teenager and young buck both are nouns.

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

As nouns, young buck is a hyponym of teenager; that is, young buck is a word with a more specific, narrower meaning than teenager:

Other hyponyms of teenager include chebab, mod, pachuco, punk, punk rocker, rocker, young man.

teenager (noun)	young buck (noun)
a juvenile between the onset of puberty and maturity	a teenager or a young adult male