2 Kings 11:4, 21

Now in the seventh year Jehoiada [the priest, Jehosheba’s husband] sent for the captains of hundreds of the Carites and of the guard and brought them to him to the house of the LORD. Then he made a covenant with them and put them under oath in the house of the LORD, and showed them the king’s [hidden] son.

Jehoash (Joash) was seven years old when he became king.