Jamieson, Fausset and Brown on 2 Chronicles 22:2

2. Forty and two years old was Ahaziah when he began to reign--(Compare 2Ki 8:26). According to that passage, the commencement of his reign is dated in the twenty-second year of his age, and, according to this, in the forty-second year of the kingdom of his mother's family [Lightfoot]. "If Ahaziah ascended the throne in the twenty-second year of his life, he must have been born in his father's nineteenth year. Hence, it may seem strange that he had older brothers; but in the East they marry early, and royal princes had, besides the wife of the first rank, usually concubines, as Jehoram had (2Ch 21:17); he might, therefore, in the nineteenth year of his age, very well have several sons" [Keil] (compare 2Ch 21:20; 2Ki 8:17).

Athaliah the daughter of Omri--more properly, "granddaughter." The expression is used loosely, as the statement was made simply for the purpose of intimating that she belonged to that idolatrous race.