Well, okay, there is still the disclaimer that everyone is over the age of 18. And they even make a joke about Monshiro's age on several occasions in S, joking about how she is supposedly over 18 even though that would make no sense. But really, she skipped several grades before she ended up a first year high school student, and is mentioned as being much younger than the rest of the cast. And, you know, she looks like a kid.