I’m not one to complain about age gaps....but why does Itou always make the younger person like 9 and the other person some ridiculously older age? Like 25 or something.
That's not the case here, I think? Anna is described as excelling in school, and in the flashback she tells their dad they're old enough to take care of themselves.
Though we never see her in a uniform, so I'm guessing just out of high school.