The word “eternal” does mean “Ages unto the ages”, but this does not mean it will end. In the Biblical usage, the term in both Hebrew and Greek always refers to endless time, or unending ages. In the Greek the plural is used, “Unto the ages of the ages” to refer to the unending ages of eternity (See Ephesians 2:7, 3:21).
God as “King of ages” refers to God as “the King of unending ages” or the “Eternal” or “Everlasting King” and refers to His dominion over all—past, present, and future—time. So when hell is said to be unto the “ages of the ages” it refers to “Unending” time.