used vs old

used is not a noun while old is a noun.

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

used (noun)	old (noun)
	past times

used (adjective)	old (adjective)
employed in accomplishing something	of long duration; not new
previously used or owned by another	(used especially of persons) having lived for a relatively long time or attained a specific age
(of persons) taken advantage of	skilled through long experience
	(used for emphasis) very familiar
	excellent
	just preceding something else in time or order
	belonging to some prior time