grownup vs somebody

grownup and somebody both are nouns.

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

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

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

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

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

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