octogenarian vs old

octogenarian and old both are nouns.

Word	Noun	Adjective	Verb	Adverb
octogenarian	Yes	Yes	No	No
old	Yes	Yes	No	No

octogenarian (noun)	old (noun)
someone whose age is in the eighties	past times

octogenarian (adjective)	old (adjective)
being from 80 to 89 years old	of long duration; not new
	(used especially of persons) having lived for a relatively long time or attained a specific age
	skilled through long experience
	(used for emphasis) very familiar
	excellent
	just preceding something else in time or order
	belonging to some prior time