We are often told that the age of the Earth isn't an important issue, since the Bible does not give the age of the Earth. This sort of argumentation is rather awkward, firstly because it's hypocritical (I can all but guarantee you this is one of the very few places they'll apply this logic), secondly because, actually, the Bible does give us a very good idea of how old the Earth is.
Obviously, the Bible does not come out and say "and at that time, the Earth was 2,000 years old". It doesn't say "thus the Lord created the heavens and the earth on the 12th day of the 9th month". It does not even explicitly say "the age of the Earth is 4,000 years". What it does do is get irritatingly specific with regard to genealogies, which allows us to calculate the age of the earth.
In Luke 3:23-38, we read "Now Jesus Himself began His ministry at about thirty years of age, being (as was supposed) the son of Joseph, the son of Heli, the son of Matthat, the son of Levi, the son of Melchi, the son of Janna, the son of Joseph, the son of Mattathiah, the son of Amos, the son of Nahum, the son of Esli, the son of Naggai, the son of Maath, the son of Mattathiah, the son of Semei, the son of Joseph, the son of Judah, the son of Joannas, the son of Rhesa, the son of Zerubbabel, the son of Shealtiel, the son of Neri, the son of Melchi, the son of Addi, the son of Cosam, the son of Elmodam, the son of Er, the son of Jose, the son of Eliezer, the son of Jorim, the son of Matthat, the son of Levi, the son of Simeon, the son of Judah, the son of Joseph, the son of Jonan, the son of Eliakim, the son of Melea, the son of Menan, the son of Mattathah, the son of Nathan, the son of David, the son of Jesse, the son of Obed, the son of Boaz, the son of Salmon, the son of Nahshon, the son of Amminadab, the son of Ram, the son of Hezron, the son of Perez, the son of Judah, the son of Jacob, the son of Isaac, the son of Abraham, the son of Terah, the son of Nahor, the son of Serug, the son of Reu, the son of Peleg, the son of Eber, the son of Shelah, the son of Cainan, the son of Arphaxad, the son of Shem, the son of Noah, the son of Lamech, the son of Methuselah, the son of Enoch, the son of Jared, the son of Mahalalel, the son of Cainan, the son of Enosh, the son of Seth, the son of Adam, the son of God."
Genealogies are the sort of thing we tend to skip when we read the Bible, and understandably so. I'm willing to bet many of you even skipped the passage above. They're not fun to read, they contain very little direct theology, it's basically an endless list of names of people, some of whom we don't even know. However, God saw fit to include them, and so we do a great disservice to both God and to ourselves when we skip over them. Of course, there is a primary purpose behind both genealogies (the other being in Matthew 1:1-17), which certainly does not include dating the earth. However, this does not mean it cannot be used to do so.
The first thing to notice is that Luke's version goes all the way back to Adam. This solidifies Adam's historicity. He is not some mythical figure, or an allegory, he is the genuine first human being in history, from whom all are descended, up to and including Jesus Christ, the "last Adam" (1 Corinthians 15:45). Luke's genealogy is not the only genealogy in the Bible. There are others, and these actually contain the age each man was when his son was born. In Genesis 5:1-5, we read "This is the book of the genealogy of Adam. In the day that God created man, He made him in the likeness of God. He created them male and female, and blessed them and called them Mankind in the day they were created. And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth. After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters. So all the days that Adam lived were nine hundred and thirty years; and he died."
The genealogy in this chapter continues to Seth, Enosh, Cainan, all the way to Noah. Now, obviously, there is a margin for error here. Just like we do, the Jews recorded age by years. On your 18th birthday, you are 18. A day before your 19th birthday, you are still 18. So there is some margin for error here. However, mathematically speaking, the difference in time between Adam's creation and the incarnation is around 4,000 years. On top of that, we know the incarnation was around 2,000 years.
So, we know how old Adam is: Around 6,000 years old. But how does that help us figure out the age of the Earth? Simple: Adam was created on the 6th day of history. If we knew Adam was created exactly 6,000 years ago today, we would know the earth is 6,000 years old + 5 days. And this is so important, the Bible explicitly states it as the foundation for other important doctrines. No one would claim the Sabbath is unimportant, and yet Exodus 20:11 tells us that creation week is the foundation for it. No one would claim marriage is enough of a side issue for us to ignore it, and yet Christ Himself said that marriage is important because God made us male and female from the beginning of creation (Mark 10:6). Christ also said that we cannot believe Him about Heavenly things if we do not believe when He speaks of Earthly things (John 3:12).
And dear brethren, that is exactly why Old Earth philosophies were invented. They create the whole "did God really say" kind of atmosphere that caused Eve to eat from the forbidden tree in the first place. Did God really say the Earth is 6,000 years old? Yes, He absolutely did. The moment you deny that, you either pave the way for someone else's destruction, or even your own. As a Church, we simply cannot afford to compromise, for sake of our own spiritual health, for sake of other believers, for sake of people we seek to witness to, and most of all, for sake of Christ.