grownup vs adult

grownup and adult both are nouns.

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

As adjectives, grownup and adult are synonyms defined as:

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

As nouns, grownup and adult are synonyms defined as:

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

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