Zedekiah was twenty-one years old when he became king, and he reigned eleven years in Jerusalem.
2 Chronicles 36:11
Surrounding context
36:10 At the turn of the year King Nebuchadnezzar summoned him and took him to Babylon, with the costly articles from the house of the LORD, and made Zedekiah, Jehoiakim's brother, king over Judah and Jerusalem.
36:11 Zedekiah was twenty-one years old when he became king, and he reigned eleven years in Jerusalem.
36:12 He did evil in the sight of the LORD his God, and did not humble himself before Jeremiah the prophet, who spoke from the mouth of the LORD.