Heaven was created when the earth was created.

"Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world." Matthew 25:34

Heaven was created sometime after the ascension of Jesus.

"In my Father's house are many mansions: if it were not so, I would have told you. I go to prepare a place for you." John 14:2

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Explanation:

The Kingdom of Heaven itself has been prepared for all mankind from the beginning of the world. That does not mean that absolutely everything within that Kingdom is already built. Jesus Christ went to prepare mansions and individual places for His disciples.

"And Jacob called unto his sons, and said, Gather yourselves together, that I may tell you that which shall befall you in the last days... Dan shall judge his people, as one of the tribes of Israel. Dan shall be a serpent by the way, an adder in the path, that biteth the horse heels, so that his rider shall fall backward. I have waited for thy salvation, O LORD." Genesis 49: 1, 16-18