How old was Zedekiah when he became king? 1 Chronicles 3:15 says Jehoiakim was born 2nd, Zedekiah was born 3rd, and Shallum [Jehoahaz] was born 4th.

Jehoahaz was 23 years old when he became king, and Jehoiakim was 25 years old when he became king 3 months later. So how is it that 11 years later Zedekiah is only 21 years old when he becomes king? Shouldn't he be 35 years old according to 1 Chron. 3:15?
From Contradicting Bible Contradictions we find an intriguing answer.
Zedekiah was a lot younger than his “position” in the line of sons of Josiah because he was actually a grandson via an affair between Jehoikim and Hamutal one of Josiah wives. So his position was immediately after his biological father Jehoikim’s position.


  Zedekiah is called: son of Josiah (verse 15), but he is also called: son of Jehoiakim (verse 16). When we take this information seriously – and we do – we have to reckon with an extramarital affair. Diagram tree of verse 15 and 16 with the mothers of the princes:
   
                                  King Josiah ￼
                ____________(spouses)________              
                |                        |                       |
  Unknown mother ￼    Zebidha ￼        Hamutal ￼ ———→
                |                        |                       |             extramarital son       
        Johanan ￼        Jehoiakim ￼     Jehoahaz ￼              |
        (firstborn)            (Eliakim)           (Shallum)         Zedekiah ￼
                                  |            —————————→ (Mattaniah)    
                             (spouse)
                            Nehushta ￼           
                                  |
                            Jehoiachin ￼
                             (Jeconia)
   
  1 Chronicles 3:15 says: Zedekiah was a son of Hamutal, a wife of Josiah (2 Kings 23:31 and 24:18).
  1 Chronicles 3:16 says: Zedekiah was also a son of Jehoiakim.
  At the age of about 15 Jehoiakim begot Zedekiah by Hamutal, one of Josiah’s spouses. In fact Josiah became the grandfather of Zedekiah and could be called his father by the rule: the son of a son is a son.
  Hamutal named the boy Mattaniah (2 Kings 24:17), meaning: ‘Gift of God’; but Josiah called him Zedekiah, meaning: ‘God is righteous’.
  Zedekiah was on the list of succession (1 Chronicles 3:15), immediately after his biological father Jehoiakim. When Jehoiakim would die as king, Zedekiah was his firstborn to succeed and that gave him the unusual position on the list of succession.