2-kings 16:2

Twenty years old was Ahaz - Here is another considerable difficulty in the chronology. Ahaz was but twenty years old when he began to reign, and he died after he had reigned sixteen years; consequently his whole age amounted only to thirty-six years. But Hezekiah his son was twenty-five years old when he began to reign; and if this were so, then Ahaz must have been the father of Hezekiah when he was but eleven years of age! Some think that the twenty years mentioned here respect the beginning of the reign of Jotham, father of Ahaz; so that the passage should be thus translated: Ahaz was twenty years of age when his father began to reign; and consequently he was fifty-two years old when he died, seeing Jotham reigned sixteen years: and therefore Hezekiah was born when his father was twenty-seven years of age. This however is a violent solution, and worthy of little credit. It is better to return to the text as it stands, and allow that Ahaz might be only eleven or twelve years old when he had Hezekiah: this is not at all impossible; as we know that the youth of both sexes in the eastern countries are marriageable at ten or twelve years of age, and are frequently betrothed when they are but nine. I know a woman, an East Indian, who had the second of her two first children when she was only fourteen years of age, and must have had the first when between eleven and twelve. I hold it therefore quite a possible case that Ahaz might have had a son born to him when he was but eleven or twelve years old.