"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. " (2 Chronicles 36:9, KJV)
The KJV follows the Masoretic reading. Most modern translators speculate that the Masoretic text is in error, seeing that 2 Kings 24:8 says Jehoiachin was eighteen years old when he began to reign. However, there is no error in the Masoretic text. Jehoiachin became co-regent with his father Jehoiakim over Judah at age eight (2 Chronicles 36:9) and became the ruler “in Jerusalem” at age eighteen (2 Kings 24:8). The young age at which Jehoiachim became co-regent is not surprising, since his father’s interest would have been to secure an heir in the face of imminent Babylonian invasion. Jehoiachin’s co-regency of ten years corresponds perfectly with his father Jehoiakim’s reign of eleven years (2 Chronicles 36:5). Moreover, as soon as the Babylonian invasion looms into the picture, Chronicles begins to use the phrase, “king over Judah and Jerusalem” (2 Chronicles 36:4, 10). The phrase is never used in Kings or in Chronicles prior to the Babylonian invasion. Prior to the Babylonian invasion, there was no need to differentiate the king of Jerusalem from the king of Judah. However, as the Babylonians came and instituted their rule, the king of Jerusalem was no longer the default king of the rest of Judah. Thus, Chronicles begins to use the phrase “king over Judah and Jerusalem” to indicate a ruler who reigned over both Jerusalem and Judah. This point is significant in regards to ascertaining the total length of Jehoiachin’s time in office. Although 2 Kings 24:8 and 2 Chronicles 36:9 say that he reigned “in Jerusalem” for three months, that does not exclude the possibility of him co-reigning over Judah for the past ten years. Since his father was the sole regent over Jerusalem under Nebuchadnezzar’s regime (2 Kings 24:1), Jehoiachin did not have authority over Jerusalem despite having co-regency over Judah. The seeming contradiction between 2 Kings 24:8 and 2 Chronicles 36:9 is a testament of the confusing political scene of the time rather than an error in the Masoretic text.
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Also read: Masoretic Readings Defended