antediluvian vs oldster

antediluvian and oldster both are nouns.

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

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

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

antediluvian (noun)	oldster (noun)
a very old person	an elderly person
any of the early patriarchs who lived prior to the Noachian deluge

antediluvian (adjective)	oldster (adjective)
of or relating to the period before the biblical flood
so extremely old as seeming to belong to an earlier period