פירוש על דברי הימים ב 34:3

when he was still a youth, he started to seek The meaning is that although he was still a youth, he started to seek after the God of David his father. This is similar to (Gen. 37:2): “and as a youth, with the sons of Bilhah, etc., Joseph brought evil reports of them,” and this is not surprising, but if he were thirty years old, he would not have done it. It is, however, surprising, that he (Josiah) was a youth, and nevertheless, he started to seek, etc.

he started to purge Judah and Jerusalem of the high places, the asherim that Manasseh had made and that Amon his son had renewed, as above (33:3): “And he rebuilt the high places that Hezekiah his father had demolished, etc.”