Junior vs younger

Junior is a noun but younger is not a noun.

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

As adjectives, Junior and younger are synonyms defined as:

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

Other synonyms of Junior include jr..

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

Junior (adjective)	younger (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