We saw from a previous article that if the Bible writer’s wanted to convey that the earth was created in six literal days around 6-8,000 years ago, they could not have been much clearer about it than they are in Genesis 1. But many otherwise fine Bible apologists sweep this option aside, even though they know the facts about the text. This is because they believe that the scientific evidence shows clearly that the Earth is billions of years old. But there are also good Bible apologists and credentialed scientists out there that hold to a young earth. Is this second group composed of all crackpots, or do they have some evidence to challenge the billion year old earth conclusion?
Before I even start this discussion, let me again emphasize that there are good Christian believers on both sides of this issue, so one can be a Christian and believe either. Most of us, myself included, were taught that the earth has been proven to be about 4.6 billion years old, and the universe even older at about 13 billion years or so. When I initially heard of the young, thousands of years old earth idea, like most others I was very skeptical. Up to that time I had never really heard of any scientifically credible evidence otherwise. So I figured that as a Bible believer, I had to reconcile Genesis to the supposed proven facts of science by interpreting the creation days as ages. But as we saw in the previous article, there are problems with this day-age idea, as well as the idea that Genesis 1 is figurative or just a myth. So I wrestled and searched, and in doing so I discovered that the proof of an old earth is not as ironclad as I first thought.
Mass spectrometer used in radiometric dating
The main proof offered of an old earth is through radiometric dating of volcanic, or igneous, rocks. (Some also point to the layers of rocks containing fossils, but I will save that discussion for my upcoming articles on the Genesis Flood.) The rates of decay of the various “parent” elements into the “daughter” elements can be very accurately measured. So if the decay rates have not changed over time, which is assumed, and the amounts of “parent” and”daughter” elements can be either assumed to be zero some other known level, then based on these assumptions the “age” of the rock can be calculated. Note that there are many assumptions and extrapolations in the process. Why? Well, none of us were there during this supposed 4 billion plus year history. Our recorded history as humans only goes back to a very Biblical 6,000 years. So we have a very large extrapolation to make in relation to our actual recorded history. Many of you who are engineers would find this level of extrapolating very uncomfortable. And our “age” derived by this method is only as good as the assumptions behind it. See some sample articles on this subject:
As we will discover, the assumptions of the Uniformitarian Model, the idea that physical processes have not varied significantly over time since the earth was formed, is a critical one in the case for an old earth. But there are other models which do not hold to this assumption. A group of PhD. scientists in the fields of geology and physics who have done research and publications in radioisotope dating came together to examine the evidence and assumptions behind the dating methods. These scientists hold to a young earth position and their research initiative was called RATE (For Radioisotopes and the Age of The Earth). They have done some ground-breaking research which reveals some of the problems with the old earth model. 
Not everyone who sees problems with using these dating methods to prove an old earth is a Bible believer. In the 1997 book Shattering the Myths of Darwinism, Author and Science writer Richard Milton, who is an atheist, points out some of the same problems creationists have raised: “…you must know the starting value of the clock-how much water was in your water clock to begin with or how tall your candle was before it was lit. And you must be sure that some external factor cannot interfere with the process while it is in operation…the process we choose will have started in pre-historic times, which we have no method of directly observing and verifying.”  He comes to this conclusion about the earth’s age: “The fact is that presently it is impossible to say with any confidence how old the Earth is, beyond the obvious fact that it predates the calendar of human history” 
For an in-depth technical analysis of the different radiometric methods and their pitfalls, a good reference is Earth’s Catastrophic Past, by Phd. geologist Andrew Snelling 
For a series of articles (somewhat technical) by a nuclear physicist Dr. Vernon Cupps, see the following:
In the relatively few cases where independent evidence exists for the age of a rock formation (such as one formed during recorded history), the radiometric ages usually turn out to be orders of magnitude greater than the true age. For example see this article:The Lava Dome at Mount St. Helens debunks dating methods
Another question to ponder is if mankind had the same brain and capabilities for 200,000 years of his supposed history, why didn’t he get around to discovering agriculture, writing, and building cities until about 6,000 years ago, again conveniently in line with the Biblical record?