Charles Ellicott Commentary 2 Chronicles 34:1

"Josiah was eight years old when he began to reign; and he reigned thirty and one years in Jerusalem." — 2 Chronicles 34:1 (ASV)

Josiah was eight years old. —So 2 Kings 22:1, which adds, and his mother’s name was Jedidah, the daughter of Adaiah of Boscath.