John Gill Commentary 2 Chronicles 33:1

"Manasseh was twelve years old when he began to reign; and he reigned fifty and five years in Jerusalem." — 2 Chronicles 33:1 (ASV)

Manasseh was twelve years old
From here to the end of (2 Chronicles 33:9) the same things are recorded, almost word for word, as in (2 Kings 21:1–9) , see the notes there. (See Gill on 2 Kings 21:1).