2 Kings 11:1, 3-4, 21

When Athaliah the mother of Ahaziah saw that her son was dead, she proceeded to destroy the whole royal family.

He remained hidden with his nurse at the temple of the LORD for six years while Athaliah ruled the land. In the seventh year Jehoiada sent for the commanders of units of a hundred, the Carites and the guards and had them brought to him at the temple of the LORD. He made a covenant with them and put them under oath at the temple of the LORD. Then he showed them the king’s son.

Joash was seven years old when he began to reign.