old vs middle-aged

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

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

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

old (adjective)	middle-aged (adjective)
of long duration; not new	being roughly between 45 and 65 years old
(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