2 Kings 8:26
26 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 Chronicles 22:2
2 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.
This is an extremely famous "contradiction" in the Bible, and even though there are many plausible solutions that have been published for over 400 years, critics of the Bible still bring this out and present it as if it's a startling revelation.
Was Ahaziah 22 years old, or 42 years old when he began to reign?
Just like in the case of Jehoiachin (read about his "contradiction" here), it appears that Ahaziah served as co-regent with his father Ahab for 20 years until his father's death.
As mentioned in the Jehoiachin article, joint-reigns were common during the period when Kings and Chronicles were written. For example, David appoints his son Solomon as king while he's still alive (1 Kings 1:33-39).
Another example would be when Jotham becomes king while his father Uzziah is still alive because Uzziah had contracted leprosy (2 Chronicles 26:21).
Oftentimes in Samuel, Kings, and Chronicles, the authors will report the same event in slightly different terms depending on their perspective and what they view is important.
In this particular instance, the author of Kings wanted to report the age that Ahaziah became co-regent, whereas the author of Chronicles saw it fit to only report the age when he became king in his own right. This is the exact same thing that they did with Jehoiachin.
Both accounts, however, state that Ahaziah reigned for 1 year, meaning they only count when he became king in his own right as his "reign".