@assuming he's seventeen and she's probably like 23/24, six, seven years isn't too bad considering how it is right now, after he turns eighteen it'll really only be wholesome
this type of story really does depend on the very specific circumstances and characters, this one's mostly just wholesome
frankly speaking, as long as it's a serious relationship based around love, even a ten year gap isn't too much
minoru is also the more mature one in many areas so i think they fit pretty well
the author obviously was aware of potential problems so he seems to have thought the setting thru to makes sure their relationship work really well