curmudgeon vs oldster

curmudgeon and oldster both are nouns.

Word	Noun	Adjective	Verb	Adverb
curmudgeon	Yes	No	No	No
oldster	Yes	No	No	No

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

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

Other hypernyms of curmudgeon include golden ager, old person, senior citizen.

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