old woman vs oldster

old woman and oldster both are nouns.

Word	Noun	Adjective	Verb	Adverb
old woman	Yes	No	No	No
oldster	Yes	No	No	No

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

Other hypernyms of old woman include golden ager, old person, senior citizen, adult female, woman.

old woman (noun)	oldster (noun)
a woman who is old	an elderly person
herb with greyish leaves found along the east coast of North America; used as an ornamental plant