How old was Jehoiachin when he began to reign?

He was 18 years old and he reigned for 3 months.

Jehoiachin was eighteen years old when he began to reign, and he reigned in Jerusalem three months. 2 Kings 24:8

He was 8 years old and he reigned for 3 months and 10 days.

Jehoiachin was eight years old when he began to reign, and he reigned three months and ten days in Jerusalem. 2 Chronicles 36:9