Hi there!
There were two youngest kings of Judah in the history of Israelites. King Joash and King Josias.
King Joash started 7 years old and King Josias started 8 years old. Both their fathers are wicked in the sight of Yahweh and reigned very shortly. But King Joash and King Josias are righteous in Yahweh’s sight because they obeyed the Law of Yahweh and implemented in the Kingdom of Judah. King Joash reigned 28 years. King Josias reigned 31 years.
There was also another young king but he is not as righteous as the former two I said. King Jehoaikin started 8 years old but reigned 3 months and 10 days and brought to exile to Babylon for he did not fear Yahweh.
King Joash and King Josias had both godly advisers during their reign. And with the help of those godly advisers, they were able to learn how to fear Yahweh at a young age.
Deuteronomy 31:13 and that their children, who have not known it, may hear and learn to fear Yahweh your God…
Proverbs 22:6 Train up a child in the way he should go: and when he is old, he will not depart from it.
Ecclesiastes 12:13 Let us hear the conclusion of the whole matter: Fear Yahweh, and keep His commandments: for this is the whole duty of man.
Its good if a child learns how to fear Yahweh at an early age because that will be his habits until he grows older.
As for me, I grew up in a Roman Catholic family and studied early on in a Roman Catholic school. If only I could start to fear Yahweh really early on, I would not have swayed in the way I should have gone and I’ll be able to experience God’s presence in my life while growing up.
But Yahweh is merciful and patient. Even if I learned how to be idolatrous at an early age like in my elementary years, Yahweh led me into being able to do this sewing craft that is biblical and honors Him. I sewed this craft when I was 10 years old. 
And it’s amazing how Yahweh called me back into His arms after so many years leading an idolatrous life because Yahweh puts people in my life that are born again Christians.
Rina ♥ 