They said she's a middle schooler in this episode. So, that's what she is, although I agree on her looking like an elementary schooler, just like her brat of a brother. The sooner Shiroe turns her down, the better. The last thing Shiroe needs to his already questionable repuration is being called a lolicon. He would have enough trouble explaining Akatsuki as it is, but at least she's of age and a level 90 assassin and could thus shut any mouths herself. Minori, however, is a kid and will be for years, nothing to explain, nothing to deny.
During the latter half of the show I started to care only about the Crusty + Princess pair, to be honest. The dynamics are so brilliant.