old vs over-the-hill

old is a noun but over-the-hill is not a noun.

Word	Noun	Adjective	Verb	Adverb
old	Yes	Yes	No	No
over-the-hill	No	Yes	No	No

old (noun)	over-the-hill (noun)
past times

old (adjective)	over-the-hill (adjective)
of long duration; not new	too old to be useful
(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