Eh, the author could well have done away with the normalcy of marriage for people "14-22". Basing your fantasy world around the past does not mean you must adhere to it completely. She could have easily been made like 18 where marriage was common for 16-25 range. This just feels like really weak justification.
How then will I continue to read this? To continue enjoying stories with questionable elements, I mentally alter how I remember the story. Based on Zerconia(autocorrect suggests "Colonizer"), we can see that the people of this world are affected significantly differently from the effects of aging. From this, we can convince ourselves that, perhaps, their brains mature earlier and they physically age much slower. Changing the biology along with the social functions that inform our understanding of childhood, we functionally change the character's effective age. In this way, Valetta goes back to being the adult I thought she was to begin with. Obviously, the author shouldn't put us in this situation to begin with, but I want to continue enjoying the work.