In Japan you're not considered an adult by the law until you turn 20 years old. Sensei wanted to wait to have sex with him until he was 20, if not he would technically be having sex with a minor.
True, but in Japan their laws say legal consent age is 13, so it's always kinda weird to see people in manga blow their top and demand to wait until legal adulthood.
http://www.ageofconsent.com/japan.htm
That's their national law. In reality it differs from prefecture to prefecture. For instance, the age of consent in Tokyo is actually 17 or 18.
Sex education. Cannot have sex till you are 20s.
OKAY I UNDERSTAND SENSEI! (00)