grownup vs fully grown

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

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

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

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

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

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