youngster vs juvenile

youngster and juvenile both are nouns.

Word	Noun	Adjective	Verb	Adverb
youngster	Yes	No	No	No
juvenile	Yes	Yes	No	No

As nouns, juvenile is a hypernym of youngster; that is, juvenile is a word with a broader meaning than youngster:

Other hypernyms of youngster include juvenile person.

youngster (noun)	juvenile (noun)
a young person of either sex	a young person, not fully developed

youngster (adjective)	juvenile (adjective)
	of or relating to or characteristic of or appropriate for children or young people
	displaying or suggesting a lack of maturity