old vs elderly

old and elderly both are nouns.

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

old (noun)	elderly (noun)
past times	people who are old collectively

old (adjective)	elderly (adjective)
of long duration; not new	advanced in years; (`aged' is pronounced as two syllables)
(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