2 Chronicles 34:3

34:3 In the eighth year of his reign, while he was still young, he began to seek the God of his ancestor David. In his twelfth year he began ridding Judah and Jerusalem of the high places, Asherah poles, idols, and images.

2 Chronicles 34:1

Josiah Institutes Religious Reforms

34:1 Josiah was eight years old when he became king, and he reigned for thirty-one years in Jerusalem.