overage vs over-the-hill

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

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

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

Other synonyms of overage include overaged, superannuated.

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

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