fully grown vs grownup

fully grown is not a noun while grownup is a noun.

Word	Noun	Adjective	Verb	Adverb
fully grown	No	Yes	No	No
grownup	Yes	Yes	No	No

As adjectives, fully grown and grownup are synonyms defined as:

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

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

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