emeritus vs old

emeritus and old both are nouns.

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

emeritus (noun)	old (noun)
a professor or minister who is retired from assigned duties	past times

emeritus (adjective)	old (adjective)
honorably retired from assigned duties and retaining your title along with the additional title `emeritus' as in `professor emeritus'	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