How old was Ahaziah when he began to rule over Jerusalem?

2 Kings 8:26 and 2 Chronicles 22:2

Twenty-two (2 Kings 8:26) - "Ahaziah was twenty-two years old when he became king, and he reigned one year in Jerusalem. And his mother’s name was Athaliah the granddaughter of Omri king of Israel," (NASB).
Forty-two (2 Chron. 22:2) - "Ahaziah was forty-two years old when he became king, and he reigned one year in Jerusalem. And his mother’s name was Athaliah, the granddaughter of Omri," (NASB). Note: the NASB corrects the copyist error and inserts 22 years. It has, however, a note saying the Hebrew states 42 years. For clarity purposes, I quoted the NASB and kept the original Hebrew number of 42.

The correct age of Ahaziah when he began to rule over Jerusalem is 22. 2 Kings 8:17 tells us that Ahaziah's father Joram ben Ahab was thirty-two when he became king and he died eight years later, at the age of forty. Therefore, Ahaziah could not have been forty-two at the time of his father's death at age forty." (Encyclopedia of Bible Difficulties, page. 206-207.)

The discrepency in ages is due to a copyist error. We can see that the difference in ages is 20 years. The system of number notation used by the Jews at the time of Ezra consisted of horizontal hooks that represented decades. would equal the number 14 where would be 24. If one or both of the hooks were smudged or flaked off of a papyri, then the dates would be off by ten years or a factor of ten.

The fact that this is a copyist error does not invalidate the inspiration or authority of Scripture. Remember, God inspired the originals. They were without error. The copies have problems, though very very few. The copies are copies of inspired documents and, unfortunately, some copyist errors did creep into the manuscripts. However, they do not affect any doctrinal areas and are very rare.