Josiah is the youngest king in the Bible. He became a king when he was 8 years old and he ruled for 31 years...
Joash was the youngest king. He was only 7 years old when he became king and he remained king for 40 years; 2 Kings 11:21 and 2 Kings 12:1.
Jehoash was 7 years old when he became King. 2 Kings 11:21
(Manasseh) I think,2 king's 21,1-3 sometimes it records a king in Chronicles briefly. King Manasseh, 12 years old. Here is a brief summary of his start: He did evil in the sight of the Lord. He rebuilt the hilltop shrines that his father had destroyed. He built altars for BaaL and made shameful Asherah idols.just as Ahab and Jezebel had done. He practiced black-majic and patronized mediums and wizards. He even set-up demonic idols ( in the temple) Because of such wicked things he led Israel into The scripture records that God would wipe away Jerusalem like a person wipes a dish and turns it upside down to dry 2 kings 21,13th verse.