The biggest weakness of Young Earth Creationism is the age of the universe and explaining how light from stars billions of light-years away got here. The evidence that this light is billions of years old seems pretty airtight. It is no coincidence that the leading proponent of Old Earth Creationism is an astronomer, Hugh Ross.
Young Earth Creationists argue that there is only the appearance of age. They claim that starlight was created in transit. It only appears old, i.e., the appearance of age.
This actually makes some sense, and it could also be argued that God made everything for a purpose. Every living thing, down to the individual cells in your body, was designed for a purpose, and starlight is the same. The purpose of the stars is to declare God’s glory, and this would not be possible if men could not see them. However, this seems to contradict Romans 1:18-20, where it says:
The wrath of God is being revealed from heaven against all the godlessness and wickedness of people, who suppress the truth by their wickedness, since what may be known about God is plain to them, because God has made it plain to them. For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse.
The passage says that the mechanism by which God’s invisible qualities can be known is creation. It is such a strong witness that men are without excuse for not believing in Him. The passage calls people wicked for suppressing the truth.
That doesn’t seem to fit people looking into the night sky and studying the universe. They are simply observing the universe and drawing logical and rational conclusions based on what they observe. The light from distant stars doesn’t just appear to have traveled billions of light-years; it has traveled that far. A logical person would have to conclude it took billions of years to get here. This is not wickedness. They are not being hostile to God. This is science, and it seems like a pretty darn good excuse to dismiss the Bible as not true.
If the universe is only a few thousand years old, then creation is lying. A rational person with no bias studying astronomy would naturally conclude that the universe is billions of years old. This is where the evidence leads. To draw any other conclusion would be giving false witness. If God created the universe in six thousand years, then one invisible quality of God that is plain would be that he is deceptive. He is a false witness.
This cannot be. An accurate creation account must account for distant starlight. To say it appears to be old is not only a weak argument but fundamentally flawed.
This is the fatal flaw of the young Earth position. The Earth is not six thousand years old. Starlight confirms it, radiometric dating (though terribly flawed) confirms it, as do the vast reserves of oil and coal composed of organic material. These are the things that kept me up at night when I was an ardent believer in a young Earth.