Responses
Naw i think its far fetched. Mental age differs from physical age. The translation 'literal child' literally means the persona's both mental and physical age is that of a child. So yes it is an error. You could be 60 with a mental age of a 15 year old, to make a point basically
so the korean raws aren’t very clear and the 2nd mugo says “don’t you know our youngest’s mental age? aren’t you ashamed?” so they way the translator did it wasn’t technically an “error” but drawing from the korean line (which isn’t that far fetched honestly). you could have two conclusions: first mugo is talking about his innocence because he is the youngest out of the 3 personalities and compared to the other two, he is a lot younger (18/19 maybe). second conclusion is he is a literal child because some personality disorders have the age of children and mugo says “mental age.” however writing something vague about his age is very iffy and the drastic translation to “literal child” is definitely a choice, glad they found that alarming as well but only the author knows what they meant when they wrote that