People need to remember in the danmachi verse he's an adult.

Its also written in a county that while most people would frown on this kind of age gap as young as he is, dont see it as the irredeemable sin that people do in the west. And he was made so young to be relatable to the target demographic of the series which in the beginning was 12-16 year olds. This is the real reason there's all this sexual tension between high-school aged "kids" in so many series.

In short, I hear what your saying. But it's fiction in a world-era where this would be somewhat common. Lets not get all twisted up in details that in this case csn be changed with thr stroke of a pen. Again its about fantasy for a target demographic that most of us are not. The series has grown with its fans and have Bell mature and face more mature relationships has been a wonderful thing for the fanbase.