Back way way long ago there was a man named James Ussher who was the Archbishop of Armagh and Primate* of All Ireland. Very important man with lots of time on his hand. So much time that he figured he'd put it to good use by counting back all of the begats in the Bible to see exactly how old the earth is. After much calculation and triangulation he came up with an exact date: Sunday October 23, 4004 BC. Nightfall, to be exact. That means that today (presumably at sunset in Israel) the earth will be exactly 6012 years old. Happy birthday, earth! You've come a long way, baby! However, I must say that you are looking awfully old for your age what with all the wrinkles, stretch marks, and radioactivity. You really need to learn to take care of yourself better.
And now for a short rebuttal:
* - Yes, it is ironic that one of the icons of Young Earth Creationism openly called himself a primate.