full-grown vs grownup

full-grown is not a noun while grownup is a noun.

Word	Noun	Adjective	Verb	Adverb
full-grown	No	Yes	No	No
grownup	Yes	Yes	No	No

As adjectives, full-grown and grownup are synonyms defined as:

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

full-grown (noun)	grownup (noun)
	a fully developed person from maturity onward

full-grown (adjective)	grownup (adjective)
(of animals) fully developed	(of animals) fully developed