old vs octogenarian

old and octogenarian both are nouns.

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

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

old (adjective)	octogenarian (adjective)
of long duration; not new	being from 80 to 89 years old
(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