Why do Biblical Creationists Emphasize a Young Earth?

There are supporters of old earth theology who insist that the Bible does not indicate the age of the earth. True, there is nothing explicitly saying that the world was created on 9 AM Oct 3, 4004 BC, or something similar. OECs (Old Earth Creationists) sometimes claim that biblical creation (YEC, or Young Earth Creation) is something new, and belief in an old earth has been the default position of Christians throughout church history. Not hardly. Belief in recent creation has been taught by the church fathers and the reformers, and getting an ancient planet or universe out of the Bible only comes through eisegesis.

Malicious Advice Mallard does not want you to learn from the sources.
Although church fathers and Reformers can help establish facts of history and offer important insight into Scripture, they were not writing Scripture itself. The opinions of people aren't as important as what Scripture teaches, and what can be reasonably inferred from the Bible. (Some owlhoots actually saddle up and ride with atheists in attacking biblical creationists! An old earth is essential for evolution. Then they claim that we are the ones being divisive for using a historical-grammatical interpretation. ) What does the Bible teach, and how should we respond to Christians who want to believe in an old earth, and claim that teaching a young earth actually interferes with presenting the gospel message?
While CMI predictably regularly receives feedback from sceptics criticizing our stance on creation, some Christians also write in who believe that creation is actually harmful to biblical Christianity. C.M. from the US writes:

The damage you people are doing to the credibility of the Christian worldview is simply incalculable. You are attempting to force upon intelligent people a false dilemma: either believe in the well-established facts of geology, or believe in YOUR unreasonable and incorrect interpretations of Gen1 and Rom 8:19–22. Correct Biblical interpretation requires that vague or ambiguous words and passages be interpreted using the light provided by clear words and passages. In Genesis chap 2 the word "day" clearly means "era." There is no sound reason to invent a DIFFERENT meaning for the same word in Gen 1. Similarly, you cannot ignore a biblical passage’s context & hope to arrive at a correct interpretation. Paul is speaking to humans ABOUT humans in Romans. What type of vicious God would punish poodles and potted plants for a human being’s sin? What kind of monster do you "Christian" saboteurs worship? May God forgive you the ENORMOUS damage you have done and will do.
You can see the response by clicking on "Does creation damage Christianity’s credibility?" In addition, I suggest reading "Is Biblical Creation a Distraction to Evangelism?"