The Bible actually never says how old the earth is, but people do infer the age by adding up the genealogy lists which give the ages of various persons in a family line. There are several places in Genesis where it lists who was the father of who, and how long they lived, so adding those up, people arrive at an age of about 6,000 years.
It has been argued that traditionally Jewish genealogies have sometimes left people out and skipped a few generations here and there when making a list.
If that happened with the lists in the Bible, then one would expect the age of the earth to actually be a bit more, but still nowhere near the 4 1/2 billion mark that the scientifically establishment usually says. (I heard a rumor they are fixing to increase it again, this time to 6 billion)
Many Bible believers, and even some (not all) Hebrew scholars have argued that the word we translated “day” in the Genesis story referred to a time period that was longer than 24 hours. (The word can mean a portion of a day, basically a full day, or an indefinite period of time depending on how it’s used.) Here, it’s used in a way that is most easily just translated “day,” as in… a regular ol’ day.
Other’s have argued there’s a gap of time in there BEFORE the seven days of creation even start. Historically, they’ve argued that this is the time the dinosaurs lived, but the Hebrew language in those verses doesn’t really allow any gap between verse 1 and 2 for the dinosaurs to live in. Some argue that the earth was covered by water for eons, in between Genesis 1:2 and 3, but you can’t fit land dinosaurs in there.
So the plain meaning of the Bible, taking a day to be basically a regular day, is that the earth is a little more than 6,000 years old. Since there was evidently no sun until day four, I think you have give a little room for God to say what is meant by “evening and morning” on those days. I think Augustine said those were “God-defined days, not solar-defined days” and I agree. Was it 24 hours, or 19 hours, or 456 hours…??
Who cares. It’s still not easy to turn it into billions of years. The bottom line is that unless you put a gap in there, or unless you take days to really mean ages, then the Bible is basically saying the earth is much, much younger than advertised.
And it IS serious about it. The Bible describes creatures that fit the description of dinosaurs in a few places (the word “dinosaurs” didn’t exist back then, so the Bible wouldn’t use that word), and the Bible describes a cataclysmic event we call “Noah’s flood” which would have left a great deal of the evidence (fossils, sediment layers, huge valleys and canyons, even perhaps continental movement) that is generally interpreted to have taken millions of years.
It really seems to come down to our starting assumptions. If evolution is where all life originated from and developed, then there MUST have been billions of years. Actually, even billions isn’t enough, but that’s another subject. If God created life and our universe from nothing, then it doesn’t matter if the earth is young or old. Any age would do. That’s why it’s easy for someone who believes God created the world, to look at evidence and say, “I think this happened suddenly,” or “I think the dinosaurs didn’t live as long ago as we once thought.” -Because God doesn’t NEED billions of years to create everything. It opens up the options so to speak.
It’s probably why dinosaur fossils with red blood cells and soft tissue have sent the evolutionary world into a tizzy.