The Bible doesn't really state how old the Earth is. The age has been inferred from the creation of Adam and his age. Which begs the question what is a day to God?
The young earther's seem to believe in an earth that is only around six thousand years old and I believe founded on a faulty misconception of the word (or length) of the biblical day as set forth in Genesis.
If you follow scripture you will read: "In the beginning, God created the Heavens and the earth".
There is no reference (or time frame) as to when or how long before he created man that this happened.
The actual time though could be billions of years, we just don't know.
(It is however, the beginning.)
Scripture doesn't say how long each creative day is in Genesis Chapter 1 but only gives the order of creation.
Genesis Chapter 2 verse 4 describes all of the creative days as one day.
The seventh 'day' is described as God's day of rest.
(I take this to mean that God is not 'creating' at this time because .. I doubt that he is tired) 8)
Paul indicated that God was still in his day of rest when he referred to the earlier words of David (ps95: 7,8,11) and to Genesis 2:2 where David urged: "Let us therefore do our utmost to enter into that rest".
(God's sabbath)
By Paul's time that day of rest was already thousands of years old.
My last reference as to a days length is at 2Pe 3:8 where Peter says: "one day is with God as a thousand years and a thousand years as one day".
I don't believe that Peter was saying God's day was actually a thousand years here, I think that he was indicating that God had no actual or true measurable length of time for him (God) as being a day .. and that God doesn't judge time as man does.