To be fair, he seems to be in his last year, so he's most likely 18. Taking into consideration the second chapter happenned in summer and they said they knew each other for more than half a year, it's more than likely they started dating when Shuuto was 18. But idk, they may know each other from before. Toshi knew Shuuto's mom, so their relationshio may have had some build up(?). The most concerning part about this is the posibility of Shuuto and Toshi being cousins I wanna believe Toshi just said it to avoid outing himself and Shuuto, but if they really are cousins, there could have actually been incest and grooming.

So he hasn’t even started college yet, meaning he is still a high school student and the relationship with this very obviously older guy has been going on a while…. K (≖_≖ )