jr. vs younger

jr. and younger both are adjectives.

Word	Noun	Adjective	Verb	Adverb
jr.	No	Yes	No	No
younger	No	Yes	No	No

As adjectives, jr. and younger are synonyms defined as:

jr. 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 jr. include Junior.

jr. (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