Your answer, "Eight", is not correct.
The correct answer to
How old was Jehoiachin when he began to reign?
is
Eighteen
as shown by
2 Kings 24:8
which is
- KJV
- 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.
- NIV
- Jehoiachin was eighteen years old when he became king, and he reigned in Jerusalem three months. His mother's name was Nehushta daughter of Elnathan; she was from Jerusalem.