1 Now it came to pass in the third year of Hoshea son of Elah king of Israel, that Hezekiah the son of Ahaz king of Judah began to reign. 2 Twenty and five years old was he when he began to reign; and he reigned twenty and nine years in Jerusalem. His mother’s name also was Abi, the daughter of Zachariah. (2 Kings 18:1-2 KJV)
Jamieson Fausset-Brown Commentary
2Ki 18:1-3. HEZEKIAH’S GOOD REIGN. 1, 2. Hezekiah … began to reign. Twenty and five years old–According to this statement (compare 2Ki 16:2), he must have been born when his father Ahaz was no more than eleven years old. Paternity at an age so early is not unprecedented in the warm climates of the south, where the human frame is matured sooner than in our northern regions. But the case admits of solution in a different way. It was customary for the later kings of Israel to assume their son and heir into partnership in the government during their lives; and as Hezekiah began to reign in the third year of Hoshea (2Ki 18:1), and Hoshea in the twelfth year of Ahaz (2Ki 17:1), it is evident that Hezekiah began to reign in the fourteenth year of Ahaz his father, and so reigned two or three years before his father’s death. So that, at the beginning of his reign in conjunction with his father, he might be only twenty-two or twenty-three, and Ahaz a few years older than the common calculation makes him. Or the case may be solved thus: As the ancient writers, in the computation of time, take notice of the year they mention, whether finished or newly begun, so Ahaz might be near twenty-one years old at the beginning of his reign, and near seventeen years older at his death; while, on the other hand, Hezekiah, when he began to reign, might be just entering into his twenty-fifth year, and so Ahaz would be near fourteen years old when his son Hezekiah was born–no uncommon age for a young man to become a father in southern latitudes [PATRICK].