2 Kings 11:4, 21

In the seventh year of Athaliah’s reign, Jehoiada the priest summoned the commanders, the Carite mercenaries, and the palace guards to come to the Temple of the LORD. He made a solemn pact with them and made them swear an oath of loyalty there in the LORD’s Temple; then he showed them the king’s son.

Joash was seven years old when he became king.