How old was Israel's youngest king, Joash of Judah, when he inherited the throne?

In a sordid story of murder and political intrigue, Joash's grandmother Athaliah (a daughter of Ahab and Jezebel) plotted to slaughter the entire royal family. Only the infant Joash was rescued and hidden. The priests staged a coup when Joash was seven, executing Athaliah and restoring David's line to the throne (2 Chronicles 24:1).

Finally, although he had been saved and enthroned by Jehoiada, the priest, and restored the Temple, Joash allowed idol worship after the death of Jehoiada.

Ultimately, Joash came under God’s judgment for murder of Zechariah, son of the priest. Then, he lost the Temple treasures when King Hazael of Aram-Damascus threatened Jerusalem. His own officers in the end killed him (2 Kings 11-13; 2 Chronicles 22:11, 24).