If you are a Christian, and you doubt the young age of the earth, do not let this concern you. You can still believe in an inerrant Word of God, and there is nothing wrong with your salvation! God created the world, young or old, and the doctrine of salvation has nothing to do with the age of the earth. Does the Bible say, “Believe in a young earth, or be condemned to hell.”? No, it doesn’t. Does it say you can lose your salvation? No, it does not. In fact, you don’t have to do anything, because your salvation is secure.
You may ask how you can believe in an inerrant Word of God and believe that the world is over 4 billion years old. You may have read all the arguments about the word “day” meaning a literal twenty-four hour period, and you can’t reconcile these two positions.
The Bible says a day to the Lord is as a thousand years to man (II Peter 3:8). Remember, Adam was created on the sixth day, and these “days” were as God sees days, because only God was here…not man. Can the word “day” be interpreted as anything other than a twenty-four hour period…I would argue yes, but in reality, it doesn’t matter how I interpret the word day…the question is, ‘what is a day to God’? If you are already convinced that the earth is old, you must come to the conclusion that a day for God is a long period of time. God is God of the universe, not just God of the earth. Day as a twenty-four hour period only applies if you are standing on planet earth. What is the meaning of day if you are in the middle of space? There is no day or night there, so why should we limit God to a limited twenty-four hour earth day?
Another objection to an old earth is that it invalidates the “death before sin” doctrine. Young earth belief is that there was no death before sin, because Adam introduced death when he sinned. This is based on Romans 5:12 and Genesis 1:29-30. Take a moment to read these verses.
Genesis 1:29-30 discusses mankind and the animals being given every green plant for food. It does not directly address death.
Of more importance is Romans 5:12, which says death entered the world thru one man (Adam). It does not specify the type of death, i.e. physical or spiritual death. Thus, you have several options here. God could have meant spiritual death only. The more viable option, however, is that yes, physical death was introduced through Adam's sin. However, this refers to humanity only. Prior to Adam, there clearly was physical death in the animal kingdom. For more, see the Death Before Sin article section.
You may have other questions that are yet unanswered. I encourage you to get some books and start reading on the subject. May God bless you as you come to a greater understanding of His creation, young or old.