Your answer, "Eighteen", is not correct.
The correct answer to
How old was Jehoiachin when he began to reign?
is
Eight
as shown by
2 Chronicles 36:9
which is
- KJV
- 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.
- NIV
- Jehoiachin was eighteen {One Hebrew manuscript, some Septuagint manuscripts and Syriac (see also 2 Kings 24:8); most Hebrew manuscripts eight} years old when he became king, and he reigned in Jerusalem three months and ten days. He did evil in the eyes of the LORD.