This inconsistency is only evident in some translations as it is often corrected.
Jehoiachin was eight years old when he became king, and he reigned in Jerusalem three months and ten days. And he did evil in the sight of the Lord. (2 Chronicles 36:9 NKJV)
Jehoiachin was eighteen years old when he became king, and he reigned in Jerusalem three months. His mother’s name was Nehushta the daughter of Elnathan of Jerusalem. (2 Kings 24:8 NKJV)
In 2 Chronicles 36:9, the NKJV is following the majority of Hebrew manuscripts. However, some Hebrew manuscripts, some Septuagint manuscripts and the Syriac have “eighteen” here as well as in 2 Kings 24:8. Most likely, the number was copied incorrectly at some point, and the error was perpetuated in subsequent copies.