Responses
I am assuming 18. At the start of the very first chapter Koichi was wearing a gakuran (high school uniform), and everyone was saying goodbye because it was the last day. So i think he was a senior in High School.
Google says in Japan they graduate high school at around 18. So he could be 18 or 19 depending on when his birthday is.
To be fair though, we don't know how much it passes between the first chapter and now, so by the time we get to this time frame, it may already be him being past 20, but he doesn't realize it because he never got to 20 in Japan so that why he says he can't drink? Idk
In Japan, when you're 20 you can drink. So I'm assuming the mc is 19 or under? What da flip