curmudgeon vs senior citizen

curmudgeon and senior citizen both are nouns.

Word	Noun	Adjective	Verb	Adverb
curmudgeon	Yes	No	No	No
senior citizen	Yes	No	No	No

As nouns, senior citizen is a hypernym of curmudgeon; that is, senior citizen is a word with a broader meaning than curmudgeon:

curmudgeon: a crusty irascible cantankerous old person full of stubborn ideas
senior citizen: an elderly person

Other hypernyms of curmudgeon include golden ager, old person, oldster.

curmudgeon (noun)	senior citizen (noun)
a crusty irascible cantankerous old person full of stubborn ideas	an elderly person