From 2 Chronicles 36:
And the people(A) of the land took Jehoahaz son of Josiah and made him king in Jerusalem in place of his father.
2 Jehoahaz[a] was twenty-three years old when he became king, and he reigned in Jerusalem three months. 3 The king of Egypt dethroned him in Jerusalem and imposed on Judah a levy of a hundred talents[b] of silver and a talent[c] of gold. 4 The king of Egypt made Eliakim, a brother of Jehoahaz, king over Judah and Jerusalem and changed Eliakim’s name to Jehoiakim. But Necho(C) took Eliakim’s brother Jehoahaz and carried him off to Egypt.(D)
5 Jehoiakim(F) was twenty-five years old when he became king, and he reigned in Jerusalem eleven years.
It appears from this text that Eliakim was actually older than the brother he replaced. That being said, why wasn't he made king in the first place?
I am aware of at least one other case in the Bible where a younger brother was made king. For example, Solomon had several older brothers (but in that case, he was explicitly designated by David as his successor). That being said, it occurs to me: what would have been "normal" for succession then? Would it have been a common occurrence for younger brothers to succeed their father? Was there any particular law on that point? (Please feel free to let me know if this should be split out into a follow-up question).