2 Chronicles 36:9
9 Jehoiachin was eight years old when he began to reign, and he reigned three months and ten days in Jerusalem: and he did that which was evil in the sight of the Lord.
2 Kings 24:8
8 Jehoiachin was eighteen years old when he began to reign, and he reigned in Jerusalem three months. And his mother's name was Nehushta, the daughter of Elnathan of Jerusalem.
This is one of the most famous alleged contradictions in the Bible.
For reference, it's addressed in the notes included in the Geneva Bible, published in 1560. And yet, almost 500 years later, we still have people throwing this at new Christians, in an effort to destroy their faith.
So, was he 8 year old or 18 years old?
He was co-regent with his father (or at the very least with his mother, who was deported with him in 2 Kings 24:12) for 10 years until his father died, and then reigned 3 months and 10 days (rounded down to 3 months in 2 Kings) on his own.
Joint reigns were quite common during the time that 2 Kings and 2 Chronicles are taking place. For example, David appoints his son Solomon as king while he's still alive (1 Kings 1:33-39).
Another example would be when Jotham becomes king while his father Uzziah is still alive because Uzziah had contracted leprosy (2 Chronicles 26:21).
Common sense also proves this, as can it be said of an 8 year old child that "he did evil in the sight of the Lord" (2 Chronicles 36:9)? How much evil can an 8 year old really be held accountable for?
There isn't a soul on the face of this earth that can demonstrate that Jehoiachin wasn't a co-regent with his father or mother for 10 years.
The author of Chronicles simply began to count him as king from the time he became co-regent, and the author of Kings saw fit to start counting when he became king in his own right. Both of them, however, only tell us that his reign lasted 3 months, meaning they only counted the time where he began to rule in his own right as his "reign".