teenager vs young man

teenager and young man both are nouns.

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

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

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

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