An excerpt from the Marriageable Age Wikipedia page:
"In ancient Rome, it was very common for girls to marry and have children shortly after the onset of puberty. Roman law required brides to be at least 12 years old. In Roman law, first marriages to brides from 12 to 24 required the consent of the bride and her father; by the late antique period, Roman law permitted women over 25 to marry without parental consent."
Of course, this specific scenario in this excerpt is about ancient Rome, so that is WAY further back than medieval times which this story's time is loosely based on.
Here's a link to the page regarding marriage and the specific excerpt in question for those curious: https://en.wikipedia.org/wiki/Marriageable_age#Antiquity
And a link to the Life Expectancy Wikipedia page as well while I'm at it: https://en.wikipedia.org/wiki/Life_expectancy#Variation_over_time
With all that said, much of this information is from Wikipedia of all things, so my information is highly susceptible to inaccuracy and misinformation. As such, take what I've said with a grain of salt.
Edit: I should also mention that many countries today still have marriageable age to be as young as 13, but it depends on where you go and whether or not there is parental and/or judicial consent.
Also, from the Marriageable Age Wikipedia, there's this: "All US states (except for Delaware and New Jersey) have exceptions/loopholes that allow minors to marry in case of parental consent, judicial consent, and/or pregnancy. Of those, 37 states have a minimum age ranging from 14-17. In 13 states, there is no minimum age. In the District of Columbia, the general age is 18 however minors aged 16 can marry with parental consent." Kinda puts some things into perspective, I guess? ¯\_(ツ)_/¯