2 Kings 23:36 (ESV)

36 Jehoiakim was twenty-five years old when he began to reign, and he reigned eleven years in Jerusalem. His mother’s name was Zebidah the daughter of Pedaiah of Rumah.

From this verse it becomes clear that Jehoiakim is the older brother of Jehoahaz, who was twenty-three when he became king. We read that, three months after the enthronement of his 23-year-old brother, Jehoiakim himself was twenty-five.