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

As adjectives, younger and Junior are synonyms defined as:

younger and Junior: used of the younger of two persons of the same name especially used to distinguish a son from his father

Other synonyms of younger include jr..

younger (noun)	Junior (noun)
	a son who has the same first name as his father

younger (adjective)	Junior (adjective)
used of the younger of two persons of the same name especially used to distinguish a son from his father	used of the younger of two persons of the same name especially used to distinguish a son from his father