elderly vs old

elderly and old both are nouns.

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

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

elderly (adjective)	old (adjective)
advanced in years; (`aged' is pronounced as two syllables)	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