adult vs somebody

adult and somebody both are nouns.

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

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

adult: 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 adult include individual, mortal, person, someone, soul.

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

adult (adjective)	somebody (adjective)
(of animals) fully developed
designed to arouse lust