old person vs old woman

old person and old woman both are nouns.

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

As nouns, old woman is a hyponym of old person; that is, old woman is a word with a more specific, narrower meaning than old person:

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