centenarian vs old

centenarian and old both are nouns.

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

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

centenarian (adjective)	old (adjective)
being at least 100 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