adult vs grownup

adult and grownup both are nouns.

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

As adjectives, adult and grownup are synonyms defined as:

Other synonyms of adult include big, full-grown, fully grown, grown.

As nouns, adult and grownup are synonyms defined as:

adult (noun)	grownup (noun)
any mature animal	a fully developed person from maturity onward
a fully developed person from maturity onward

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