junior vs immature

junior is a noun but immature is not a noun.

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

junior (noun)	immature (noun)
the younger of two persons
a third-year undergraduate
term of address for a disrespectful and annoying male

junior (adjective)	immature (adjective)
younger; lower in rank; shorter in length of tenure or service	not yet mature
used of the third or next to final year in United States high school or college	characteristic of a lack of maturity
including or intended for youthful persons	(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