Jehoahaz was originally named Shallum at birth (see study note on 1 Chronicles 3:15). His throne name, Jehoahaz, has been found on seals from the 600s BC. He became king at twenty-three, while his brother Jehoiakim was twenty-five (2 Kings 23:36). There is no explanation for why the younger brother became king.
Jehoahaz reigned for three months in 609 BC, possibly during the time when Pharaoh Neco was trying to help the Assyrians (2 Kings 23:29).