Responses
This is the official translation by June, which means that their ages were probably changed to be more suitable for Western audiences. I think they are actually graduating from middle school. If you think of this story happening to a middle schooler (12-13) rather than a high schooler (17-18), the characters, their maturity, and the level of abuse makes a whole lot more sense.
Wait, they only started having sex after they graduated, why is Yuuchan saying his voice is only changing "now, at age 15"? He should be college age????