Adam Clarke on 2 Kings 23:31

Jehoahaz was twenty and three years old - This was not the eldest son of Josiah, which is evident from this, that he was twenty-three years old when he began to reign; that he reigned but three months; that, being dethroned, his brother Eliakim was put in his place, who was then twenty-five years of age. Eliakim, therefore, was the eldest brother; but Jehoahaz was probably raised to the throne by the people, as being of a more active and martial spirit.