immatureness vs immature

immatureness is a noun but immature is not a noun.

Word	Noun	Adjective	Verb	Adverb
immatureness	Yes	No	No	No
immature	No	Yes	No	No

immatureness (noun)	immature (noun)
not having reached maturity

immatureness (adjective)	immature (adjective)
	not yet mature
	characteristic of a lack of maturity
	(used of living things especially persons) in an early period of life or development or growth
	(of birds) not yet having developed feathers
	not fully developed or mature; not ripe