old woman vs senior citizen

old woman and senior citizen both are nouns.

Word	Noun	Adjective	Verb	Adverb
old woman	Yes	No	No	No
senior citizen	Yes	No	No	No

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

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

old woman (noun)	senior citizen (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