One of the great divides in the Christian world today revolves around the age of the earth. There are two groups represented here, those who hold to an age that is relatively young, geologically speaking, where the earth would be between 6 to 10,000 years old (young earth view) and those who say the earth is 4.6 billion years old (old earth view or progressive creationism). Some in the young earth camp may say that the earth is up to 50,000 years old, but most would adhere to an earth no older than 10,000 years old.
The divide is complex, but revolves around two key points. Point one is how the Bible is to be interpreted and the second point is how do we interpret science. It should be pointed out that both groups are referred to as Christians. Also, it is important to note that the age of the earth is not an essential Christian doctrine or even a point that salvation is contingent upon. This, however, does not diminish the friction that exists within the Christian community.
Two of the primary concerns of the young earth creationist deals with how the days of creation are to be interpreted and the problem of Darwinian evolution. From the young earth interpretation, the days of creation are a literal 24 hours. On this view, interpreting the days as long time periods or slipping a long gap of time in creation might lend support to a Darwinian viewpoint. Because of this, young earth creationist see any Christian who does hold to an old earth view as being unbiblical and leaving room for Darwinian evolution.
I believe there is room for both views, though I hold to a progressive view. What can't take place is for one side to label the other. The labeling I am referring to involves calling one side a non-Christian just because they don't see your view as you see it. It should be noted that the Bible makes no statement concerning the age of the earth. Also, those who hold to a progressive view (who are not theistic evolutionists) do not accept Darwin's view of evolution.