It's not exactly 20. The TL;DR is this. The age of majority depends on some factors. 15/16 is about the age where you finish Middle School and then can move onto: Working, Trade School, High School, or take a break. From what my friend said working in the school system there, a number of his Middle School students decided to go into working jobs rather than pursuing High School, despite the last year of Middle School is enlarge preparation for High School entrance exams. 18 is where they consider you legal for the adult industry and among other things (including sexual activity). The marriage laws are a bit weird and I know there's been attempts to revise them, but the situation has led to infighting, if I recall.
20 is the final part of the age of majority, which includes drinking, smoking, and such. Think of this similar to the US system in that particular regards. Majority of the prefectures are 17/18 for age of consent, that said despite the national age of consent being 13, with only a few as low as 15 (to my knowledge). Not that I will ever defend the lowest age range on the prefectures (including the fact the national age of consent has not been updated either), but I wanted to note the general context before anyone tries to bring this up.
Regardless, in no way what the writer did is right, especially given the age of the girls enlarge is 12-15. It wouldn't make it any better if they were of legal age, and in fact makes the act worse and disgusting the fact he'd pray on women who were still minors. Like, what the fuck.
Also, to the other poster(s) in this thread: please do not use the "What is the legal age" to even try to make this better. It isn't. It will never be. And in fact, tries to downplay the seriousness of this.