old vs used

old is a noun but used is not a noun.

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

old (noun)	used (noun)
past times

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