younger vs junior

younger is not a noun while junior is a noun.

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

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

younger (adjective)	junior (adjective)
used of the younger of two persons of the same name especially used to distinguish a son from his father	younger; lower in rank; shorter in length of tenure or service
	used of the third or next to final year in United States high school or college
	including or intended for youthful persons