How old was Jehoiachin when he became king of Judah, 8 or 18?

Both 2 Kings and 2 Chronicles agree that Jehoiachin was king of Judah when Jerusalem fell to the Babylonians. They are less consistent, however, when it comes to his age at the start of his reign.

According to 2 Kings, Jehoiachin was 18 at his accession:

Jehoiachin was eighteen years old when he began to reign… [2 Kings 24:8a (NRSV)]

According to 2 Chronicles, though, he was only 8:

Jehoiachin was eight years old when he began to reign… [2 Chronicles 36:9a (NRSV)]

So how old was Jehoiachin when he became king of Judah?