Because there are numerous commentators who assume that Azariah became king (in a co-regency) after his father had been imprisoned, there are also translations that place this text in the past tense: “The people of Judah elected Amaziah’s sixteen-year-old son to be the successor of his father.” That the subject of this sentence is “the people” also indicates this; after all, who else was to appoint the successor now that Joash celebrated his victory and Amaziah was his captive? You can read more about Azariah in 2 Kings 15:1–7.
21 And all the people of Judah took Azariah, who was sixteen years old, and made him king instead of his father Amaziah.