Question: King Jehoiachin is mentioned in 2 chr. 36.9 as being 8 years old when he began to reign, but in 2 kings 24.8 he is mentioned as being 18yrs. old! Can you reconcile the two verses!
Response: One of the common explanations is that Jehoiachin was 8-years old when he began to reign as a co-regent king with his father, and that he was 18-years old when he reigned alone as king.
If this is true, then the passage in 2 Chronicles 36:9 is referring to when Jehoiachin began to reign with his father as co-regent, and the passage in 2 Kings 24:8 refers to when Jehoiachin became king.
During ancient times, there were occasions when a king would allow his young son to become a co-regent, meaning that the son would also be king while his father continued to be king, so that the young son could learn how to govern from his father.
Another explanation involves the possibility of a copyist error. Some ancient manuscripts of 2 Chronicles 36:9 say that Jehoiachin was eight years old when he became king and other ancient manuscripts say that Jehoiachin was eighteen years old when he became king. That could mean that a copyist made a mistake when copying the age of Jehoiachin.