2 Chronicles 34:3
For in the eighth year of his reign, while he was yet young, he began to seek after the God of David his father: and in the twelfth year he began to purge Judah and Jerusalem from the high places, and the groves, and the carved images, and the molten images.
While still young, Josiah began to seek after God. That seeking began in the eighth year of his reign. His seeking meant consecrating and devoting himself fully to God's service, showing he was not indifferent to the prevailing idolatry.
As king, he used his position to advance the principles of God's law. In the twelfth year, four years later, he began to purge Judah and Jerusalem of idolatrous high places, groves, and carved and molten images. He was twenty years old when he began this purging work.
The purge was an earnest effort to remove temptation from his subjects and keep them from temptation. His early decision and commitment to seek God led into active reform aimed at removing idolatry and turning Judah toward God. The reading of the law in Josiah's day is connected with a deep work of repentance and return to the Lord.