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, 2nd 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, 2nd Chronicles 22:2.
So, how old was Ahaziah when he began to reign? Most commentators go back twenty years before his actual birth and reckon him to be in his Father's loins, or they attribute him to being in his mother's egg. Bishop Ussher who labored diligently to reconcile all of the earth's history with the bible and whose dates are still used in the Oxford Text of the King James Bible harkened back to his mother.
Such a reckoning has some justification in the bible. The Apostle Paul attributed Levi paying tithes when in actuality it was Abraham who had done so. And as I may so say, Levi also, who receiveth tithes, payed tithes in Abraham, Hebrews 7:9. Likewise, all of Israel is said to have passed between the parts of the calf wherein it was Abraham who had done so hundreds of years earlier(Genesis 15:9-17). And I will give the men that have transgressed my covenant, which have not performed the words of the covenant which they had made before me, when they cut the calf in twain, and passed between the parts thereof, Jeremiah 34:18.
I have no quarrel with men who reconcile the two given ages for Ahaziah's rule by attributing the age of 42 to be reckoned from a time prior to his birth. Such an argument can be supported from within our King James Bible without doing violence to the text. Nevertheless, I see another avenue.
To understand that approach we need to consider the two books wherein Ahaziah's age is given and consider how each book sees the reign of a given king. As I discuss in my book Further Thoughts on the Word of God: Revised Edition, The two books labeled 1st and 2nd Samuel respectively view the lineage of the Kings and their right to rule through the bloodline of David. The two books labeled 1st and 2nd Chronicles view those same events through the place of the Tabernacle and the Temple.
1st Samuel is also labeled the 1st Book of the Kings. There are two kings over Israel in that book, Saul and then David. The book pivots around the very middle verse of that book; And he sent, and brought him in. Now he was ruddy, and withal of a beautiful countenance, and goodly to look to. And the LORD said, Arise, anoint him: for this is he, 1st Samuel 16:12.
1st Samuel has 31 chapters making chapter 16 to be the middle chapter. Chapter 16 has 23 verses making verse 12 the middle verse. Half of the book has Saul as the anointed king, in the middle verse it pivots and the last half of the book has David as the anointed king. Throughout the four Books of the Kings, 1st Samuel, 2nd Samuel, 1st King, and 2nd Kings, the bloodline of David is the focus.
If we apply the same test to 1st Chronicles, the middle verse is 1st Chronicles 15:15; And the children of the Levites bare the ark of God upon their shoulders with the staves thereon, as Moses commanded according to the word of the LORD. To read 1st Chronicles is read how that the Lord raised up David to restore the Tabernacle to its place, to get the plans for the Temple, and to provide the material for its construction. The middle verse places the ark of God in its right place, the right way by, the right people, by the will of David.
In a nutshell, the four Books of the Kings look at Israel's history in relation to the lineage of David, whereas the two Books of the Chronicles views Israel's history through the Kings" relationship with the House of God. Both David and Solomon were crowned as king more than once.
What shall we say then in regards to Ahaziah King of Judah? We know that he only reigned one year. Both accounts agree to that. We know that his mother Athaliah is the first runner up in the most wicked queen category only being surpassed by her mother, Jezebel. Both accounts agree there. Is it feasible that Ahaziah could actually exercise any authority as long as his mother lived? It was not in the nature of either Jezebel or Athaliah to allow one of their children to rule.
What if we view Azariah's first age given in the Book of 2nd Kings as his age when his descent from David made him the titular king? Given the track record of his grandmother Jezebel and viewing the murderous coup that his mother performed upon his death, it is doubtful that anything he said bore any weight in the running of the kingdom.
But, what about his relationship to the Temple of God? Therein we find a couple of tidbits. We know that about 20 years after Ahaziah's reign, his son Joash made it his project to repair the long neglected House of God; And it came to pass after this, that Joash was minded to repair the house of the LORD, 2nd chronicles 24:4. We also find out that his father Ahaziah had dedicated precious gifts to the House of God. And Jehoash (the same person as Joash)king of Judah took all the hallowed things that Jehoshaphat, and Jehoram, and Ahaziah, his fathers, kings of Judah, had dedicated, 2nd Kings 12:18.
It is highly unlikely that any gifts that Ahaziah had given to the House of God during his reign and under the influence of his mother would have been used in the House of God. It wasn't until 20 years later that the gifts that he had dedicated finally were utilized where he wanted them used. Perhaps this was the one and only edict of his miserable year sitting on a useless throne and being henpecked by his mother that was actually carried out.
If so, it would explain why the Book which reckons a king by his relationship to David sees him as beginning to reign at 22 years of age, and the book that reckons a king according to his relationship to the House of God and the promises of God sees him as being 42 years of age. Twenty years after his physical ascendence to the throne, his first and probably only act as a king was put into place long after his death.