When I say I am 36 years old, it means I have lived in the world for 36 years. But, God does not live in the world. Instead, the world lives in God. As St. Paul told the Athenians in Acts 17:28, “In Him we live and move and have our being”.
We have an age because we live in the world, and the world has an age because the world lives in God. But God does not live in anything or anyone else. God has always been.