I've been searching through both the setting and the FAQ page for an answer for this with no luck (plus I'm fighting off a really bad case of allergies which is kind of messing with my brain right now) so if I've somehow missed this being answered already I deeply apologize!

Is there any particular reason for the restriction on characters under 16? (Though if it turns out it's for spoilery in-game reasons, it's fine not to get too into it.)