old vs centenarian

old and centenarian both are nouns.

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

old (noun)	centenarian (noun)
past times	someone who is at least 100 years old

old (adjective)	centenarian (adjective)
of long duration; not new	being at least 100 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