I'll just go about these questions randomly. It's more interesting that way.
This question has two parts to its setup. The first is obviously "If the Earth is 6000 years old.." The second part of the setup is "and God is eternal and has always existed..." Now, here's the question.
Given those two suppositions by some fundamentalist Christians, then it follows that God has been sitting around for all eternity (because half of eternity is still eternity, correct? Half of eternity is behind us and the other half is always in front of us. That never changes), all by himself, in the dark (because he created light 6000 years ago, right?), all alone. And then one day, 6000 years ago, he decides to create Earth and the entire universe that goes along with it. Just out of the blue. Why then? What was God doing for the rest of that time? Planning it all? But if God is perfect and all-knowing, then He already knew everything that He must do, and He even knew when He was going to do it! So, why, out of the darkness and emptiness, in all that "waste and void", after spending eternity in that darkness, did He decide that 6000 years ago was the correct time to make Heaven and Earth?
That makes absolutely no sense to me.