16 seems the most appropriate. Perhaps it is worth looking at...
https://en.wikipedia.org/wiki/Age_of_con..._attitudes
https://en.wikipedia.org/wiki/Marriageab..._attitudes
Level of technology has very little to do with social attitudes, as well. Even in the 19th century, it was still pretty damned low. We shouldn't wrinkle our nose at the possibility that the age of majority being less than what we're comfortable with in real life, because Eorzea is not real life, and closer to history.
https://en.wikipedia.org/wiki/Age_of_con..._attitudes
https://en.wikipedia.org/wiki/Marriageab..._attitudes
Level of technology has very little to do with social attitudes, as well. Even in the 19th century, it was still pretty damned low. We shouldn't wrinkle our nose at the possibility that the age of majority being less than what we're comfortable with in real life, because Eorzea is not real life, and closer to history.