over-the-hill vs overage

over-the-hill is not a noun while overage is a noun.

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

As adjectives, over-the-hill and overage are synonyms defined as:

Other synonyms of over-the-hill include overaged, superannuated.

over-the-hill (noun)	overage (noun)
	a surplus or excess of money or merchandise that is actually on hand and that exceeds expectations

over-the-hill (adjective)	overage (adjective)
too old to be useful	too old to be useful