I think some of it is how you preface it. 18 is the age of legal consent and marriage in many US states, but modern sensibilities do tend to think that while it's okay for kids that age, or even a bit younger, to experiment sexually with each other, they shouldn't marry and start families until older. We also tend to take a dimmer view of men who are in their thirties and forties who prefer girls who are just barely "legal" rather than more emotionally mature women at least a bit closer to their own age. Lots of reasons for that, but of course, they don't always apply in archaic cultures.
BUT at various times in the old days, people, especially girls, and especially royals of both sexes, often married younger (and there was often a marked age difference between participants in noble and royal marriages). Of course, if your story is a fantasy, it doesn't have to be a copy of the middle ages in all respects, or even most. Many modern fantasy writers have worlds with more gender equality, for instance, than most places in "real" history. But it's your world and your story.