Age of the ages, because there will come a time when Jesus will hand it all back over to the Father and God will be all in all.
Even though Jesus is eternal and lives forever more, He reigns until the last age, then hands it back to our Father, hence the ages.
(Rev 1:17) And when I saw Him, I fell at His feet, as dead. And He put His right hand on me, saying to me, Do not fear. I am the First and the Last,
(Rev 1:18) and the Living One; and I became dead; and, behold, I am living forever and ever. Amen. And I have the keys to hell, and of death.
I am not an expert in this, but it seems to me that these are comforting words to the elect. When we spiritually see Him, we fall to His feet and die of this carnal mind. Then He comforts us, by saying, " Do not fear, for I also became dead, yet I am living forever, and have the KEYS to hell and of death".
Keys are used to open doors, and just who is the door? And there is only one way through this door. It all points to Christ, the One who we follow. He is the door, and has the keys, and is the gatekeeper.
God the Father and the Christ, our Lord and Saviour, live in the heavenlies, or eternal realm not bound to time as we are. When time is put on them, it does not matter if it is ten years, one thousand years, ages or age of the ages, they are still eternal.
This is the way I see it, could be wrong. Would like other input on this to make sure.
God bless,
Gary