How old was Jehoiachin when he became king of Judah?

In the book of  2 Kings 24:8 and 2 Chronicles 36:9, we can see the 'contradiction' you are pointing out. 2 kings says "Jehoiachin was eighteen years when he began to reign" while 2 Chronicles says he was eight years. We can't say for sure which one of them was wrong, but note that Bible manuscripts were handwritten and due to human error, someone could have written eight instead of eighteen and vice versa. It could also be that Jehoiachin started ruling alongside his father while he was 8 and only took full control over the kingdom when his father died (maybe ten years later when he turned 18).