
I wanted to like this but that age gap is a no no

He's a senior in high school which means he's 18 years old. In many countries (mine included) when you turn 18 you're legally an adult, meaning you can get a driving licence, go to bars, buy alcohol etc. You also have to remember that age doesn't correlate to one's maturity and should never be treated as such. Everyone experiences life differently and Riku is definitely mature for his age, since he's had to take way more responsibility than a normal high schooler. To be honest, he has the kind of emotional maturity I wish I had when I was 18.

It is, but it's not as simple as that. From Japanese Ministry of Justice: "Article 176. (Forcible Indecency) - 'A person who, through assault or intimidation, forcibly commits an indecent act upon a male or female of not less than thirteen years of age shall be punished by imprisonment with work for not less than 6 months but not more than 10 years. The same shall apply to a person who commits an indecent act upon a male or female under thirteen years of age.'"
BUT! "The Children Welfare Act chapter 34 forbids any act of 淫行 (which can mean fornication, harlotry or obscenity) with children (here defined as anyone under 18 years of age)."
Hope this clears it up.

Of course there is a point. 13 is the age of consent by law, Children Welfare Act 34 acts for protection for children. They're both used and don't rule each other out. It just means that the situations differs depending on the ages of both parties involved.
For example if both parties are under 13 years of age they cannot be punished, where as if the victim is under 13 and the perpetrator is under 18 they're tried as juveniles for felony. 20 years old would be treated as an adult under Juvenile Court and if the perpetrator is over 20 years old it's a felony. So if the victim is under 13 and the perpetrator is 14 or older it's a felony, but treated in different Courts based the age of the perpetrator.
Now, if the victim is 13 or older (but under 18) and the perpetrator is over 20 it's a misdemeanour. If the perpetrator is under 20 years old it depends on the legal interpretation, but can be tried as a juvenile misdemeanour.

Woops, just I noticed I made mistakes while writing this, sorry. (Never multitask, people!) It should be
"For example if both parties are under 13 years of age they cannot be punished, where as if the victim is under 13 and the perpetrator is under 18 they're tried as juveniles for felony. Under 20 years old would be treated as an adult under Juvenile Court and if the perpetrator is 20 years old or older it's a felony. So if the victim is under 13 and the perpetrator is 14 or older it's a felony, but treated in different Courts based on the age of the perpetrator."
This news is absolutely devastating. They were such a fantastic artist and author that it kills me to think they were taken so young. Rest In Peace peyo sensei :((