In order to have a "second Adam" - Jesus Christ, you have to have a "first Adam." That means a young earth, because Adam and Eve would have been lost in the millennia and the records would be inaccurate.
"Thus it is written, “The first man Adam became a living being”; the last Adam became a life-giving spirit. [SUP]46 [/SUP]But it is not the spiritual that is first but the natural, and then the spiritual. [SUP]47 [/SUP]The first man was from the earth, a man of dust;the second man is from heaven." 1 Cor. 15:45-47 ESV
I really like the way the Message puts this:
"We follow this sequence in Scripture: The First Adam received life, the Last Adam is a life-giving Spirit. Physical life comes first, then spiritual—a firm base shaped from the earth, a final completion coming out of heaven. The First Man was made out of earth, and people since then are earthy; the Second Man was made out of heaven, and people now can be heavenly. In the same way that we’ve worked from our earthy origins, let’s embrace our heavenly ends." 1 Cor. 15:45-49
I have lots of scientific reasons I also believe in a young earth, created by God in 7 days, but I will stick to the Bible here.