II Kings 11:1, 3-4, 21

When Athaliah the mother of Ahaziah saw that her son was dead, she arose and destroyed all the royal heirs.

So he was hidden with her in the house of the LORD for six years, while Athaliah reigned over the land. In the seventh year Jehoiada sent and brought the captains of hundreds—of the bodyguards and the escorts—and brought them into the house of the LORD to him. And he made a covenant with them and took an oath from them in the house of the LORD, and showed them the king’s son.

Jehoash was seven years old when he became king.