Q) How do I defend the Bible against people that say humans are millions of years older than just 6000 years believed by Christians?
A) This is a great question.
There are two Christian schools of thought on this.
1) The Bible can not be taken literally, and the ages of the characters in the Bible, and the 6 days of creation in Genesis 1, are all metaphorical. Therefore there is no way to get an accurate literal estimate of the earth’s age or the dawn of humanity.
2) The Bible can be taken literally, and the common interpretations of the scientific data is not compatible with the Biblical world view.
You can be a Christian and hold to either of these views. However I hold to the second view which is that the Bible is reliable, able to be taken literally, and not in error. I have read that when carbon dating is done, which is the primary scientific means of dating ancient organic matter, about %90 of the data that is acquired is thrown away. So when the process is done, the analyst will throw away 9 out of 10 of the results, and keep only the one. It is no coincidence that the one of out ten results they get is compatible with an old earth worldview, and the nine out of ten results that are thrown away are compatible with a Biblical worldview of the age of the earth.
When we can not trust our scientists to objectively explore data without bringing their world view into what they chose to accept and reject, we can not trust the information given to us by them.
I believe in science, and I believe that science is one of the most fun ways to worship our God. I reject any information, be it religious or secular, that is biased and lacks objectivity.
I have read from a number of respected Christian scholars who hold to the old age worldview, and I have read from a number of scholars who hold to a more literal Biblical interpretation of the age of the world. Either is fine, because these are not salvation issues. Believing in one or the other will not secure or destroy your salvation. But I figure that when we start to reject any part of the Bible as literal, it is a slippery slope from a non-salvation issue to eventually a salvation issue. It may start with a non-essential doctrine of seemingly minimal consequences, but this is probably an open door for an eventual detraction from more essential doctrines. Therefore I will side with the Bible over any man made discovery any day, and place my faith in the Word of God to eliminate any potential for spiritual attack later.