“Two and twenty years old was Ahaziah when he began to reign; and he reigned one year in Jerusalem. And his mother's name was Athaliah, the daughter of Omri king of Israel.” 2 Kings 8:26
“Forty and two years old was Ahaziah when he began to reign, and he reigned one year in Jerusalem. His mother's name also was Athaliah the daughter of Omri. He also walked in the ways of the house of Ahab: for his mother was his counsellor to do wickedly.” 2 Chronicles 22:2
The focus of these two accounts should be on the short one-year length of Ahaziah’s wicked reign and on the evil counsel his mother Athaliah the “daughter” (or as both Kings and Chronicles document, actually the grand-daughter of wicked Omri). Athaliah taught Ahaziah wickedness from his infancy in that she supported the worship of Baal. Here the accounts are in full agreement.
The words “years old” are supplied in the translation and there are two possible explanations of the variant readings. One explanation is that 2 Chronicles 22:2 is tracking the history of the godless house of Omri which at this point had exercised power 42 years. This is the explanation given in the Seder Olam, a venerable Jewish commentary. The second explanation is that is a scribal error rendered forty where it should have been twenty. It seems most reasonable that Ahaziah was 22 years old and his treacherous murdering mother around forty years old.