grownup vs someone

grownup and someone both are nouns.

Word	Noun	Adjective	Verb	Adverb
grownup	Yes	Yes	No	No
someone	Yes	No	No	No

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

grownup: a fully developed person from maturity onward
someone: a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

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

grownup (noun)	someone (noun)
a fully developed person from maturity onward	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

grownup (adjective)	someone (adjective)
(of animals) fully developed