juvenile person vs someone

juvenile person and someone both are nouns.

Word	Noun	Adjective	Verb	Adverb
juvenile person	Yes	No	No	No
someone	Yes	No	No	No

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

juvenile person: a young person, not fully developed
someone: a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

Other hypernyms of juvenile person include individual, mortal, person, somebody, soul.

juvenile person (noun)	someone (noun)
a young person, not fully developed	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential