Though this is fantasy, it's still a pre-industrial world with swords and bows. An 18 years old woman marrying a 27 years old man would have been more than ordinary in the past. Women tended to be sent off to marriage young, whereas the men very often were older, so that they would have had time to land a decent job and make some money for the new family. A young lad wouldn't yet have much financial prospects, unless he was from a well-off family and more importantly the one inheriting the family business/position.
Of course in the case of poor families (which were the vast majority of families centuries ago), there wasn't necessarily much in the way of options and a young woman would be married off to anyone, so that she wouldn't burden anymore her parents, who might have a bunch of younger children to feed. If she had got a job as a maid or as an apprentice somewhere (and thus also a new guardian), she could potentially buy some time for herself, to land a husband of her liking.
Young wives had more time and health to bear children. You typically needed a lot of children because it wasn't that uncommon for more than half to die before adulthood, to common diseases today only experienced by the anti-vaxxers.