How long has God been alive?

Always.

God has no beginning and no end. God created the very concept of time. As humans, we think of everything in terms of how long we have been alive, but as hard as it is to understand, God has never not existed. He is forever (past, present and future).

God is eternal.

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Deuteronomy 33:27 — "The eternal God is a dwelling place, And underneath are the everlasting arms; And He drove out the enemy from before you, And said, `Destroy!'

Jeremiah 10:10 — But the LORD is the true God; He is the living God and the everlasting King. At His wrath the earth quakes, And the nations cannot endure His indignation.

Psalm 90:2 — Before the mountains were born Or You gave birth to the earth and the world, Even from everlasting to everlasting, You are God.

Rev 1:8 — "I am the Alpha and the Omega," says the Lord God, "who is and who was and who is to come, the Almighty."

Rev 22:13 — ~"I am the Alpha and the Omega, the first and the last, the beginning and the end."

Psa 90:2 — Before the mountains were born Or You gave birth to the earth and the world, Even from everlasting to everlasting, You are God.

Hab 1:12 — Are You not from everlasting, O LORD, my God, my Holy One? We will not die. You, O LORD, have appointed them to judge; And You, O Rock, have established them to correct.

Psa 93:2 — Your throne is established from of old; You are from everlasting.