middle-aged vs old

middle-aged is not a noun while old is a noun.

Word	Noun	Adjective	Verb	Adverb
middle-aged	No	Yes	No	No
old	Yes	Yes	No	No

middle-aged (noun)	old (noun)
	past times

middle-aged (adjective)	old (adjective)
being roughly between 45 and 65 years old	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