juvenile vs youngster

juvenile and youngster both are nouns.

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

As nouns, youngster is a hyponym of juvenile; that is, youngster is a word with a more specific, narrower meaning than juvenile:

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

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