Okay, I'm pretty sure our second-year high schooler is old enough to consent, even if he isn't legally an adult. Considering Asahi hasn't thought about sex being a part of dating, this probably isn't actually a huge deal. But legally, it's probably fine, and y'all should calm down. (Your morals may vary.)
If anyone is at fault here, it's probably Asahi, because he didn't put any thought into what it meant to date someone older while still in high school. (Maybe because his friends are...not great role models when it comes to that.) He was the one who ended up pursuing Kanefumi and asking him out, despite being fully aware there was an age gap between them! It was clear from the first time they actually talked to each other: Asahi sees Kanefumi's backpack full of books and asks, "Are you a college student or something?" Kanefumi says, "Yeah, I'm carrying a lot of books. I write essays, so they're convenient to have around." It's a little ambiguous if the response was confirmation of being a student or just being like a student. Personally, I had been under the impression Kanefumi was still in college.
All Kanefumi knew was that while Asahi was younger than him, he was clearly old enough to work nights at a convenience store. He may also have seemed more mature because he's tall and has a lot of rizz and an extensive dating history. Asahi's casual clothes are also pretty stylish - he doesn't look out of place next to Kanefumi at all and comes across as a 'popular' guy who gets attention from more than just teenage girls. By the time he thinks to ask Asahi's age, it's during a meteor shower, and they get distracted by shooting stars before he hears the answer.
So could Kanefumi have asked Asahi's age sooner? Probably. But he probably didn't see a reason to, because an age gap is less important than having someone you like like you back?.. (I really don't think his ex being a salaryman was meant to signal that Kanefumi is dramatically older than he appears, the dude could have been anywhere from 22-30.) I think that age gaps aren't quite as big a deal in lgbtq circles. It's hard enough to meet people who are 'out' in your own city, let alone exactly your age. (Or so my friends have told me.)
I think it's more interesting that Kanefumi is very clearly Not Okay with Asahi being in high school when he finds out, because that did not seem to be an option he had considered at all. Maybe it's against his own morals/ethics, even if it isn't technically illegal. Or maybe it's another lesson for Asahi to learn: no matter how grown up you seem, dating a fellow teenager isn't the same as dating an adult, so be more mindful about flirting?
In a typical BL manga, they would happily date and Kanefumi would just be the older, experienced one, and the story would probably end with a proposal at graduation. I suspect this story is not going to go that way. Maybe this is a relationship that won't work out because the timing is wrong, and it'll be one of those fleeting, seasonal memories they talked about during the meteor shower.
