2 Kings 11:1, 3-4, 21

And when Athaliah the mother of Ahaziah saw that her son was dead, she arose and destroyed all the seed royal.

And he was with her hid in the house of the LORD six years. And Athaliah did reign over the land. And the seventh year Jehoiada sent and fetched the rulers over hundreds, with the captains and the guard, and brought them to him into the house of the LORD, and made a covenant with them, and took an oath of them in the house of the LORD, and shewed them the king's son.

Seven years old was Jehoash when he began to reign.