@mirewitch The thing is, that didn't happen, Meguru literally doesn't explain anything about what being gay is, their initial interactions was basically Meguru trying to understand wth was going on, and why Kei was using that uniform (its literally in the desc of the story), Kei obviously knows what being gay is, hell, he came out to his mother, he simply didn't had anyone that understood that he simply liked other guys (since his mother basically linked "linking guys = being a girl"), the only time we see Kei ask anything sexually related is wen he asks Meguru if he has had sex, which later on we can obviously see this action as him asking not because he doesn't know, but because he bloody likes the dude, Meguru simply saw this as him being curious obviously.
Now, if you wanna go whit realism, despite this obviously being a story only meant to introduce the topic of discussion and not to portrait it correctly , obviously, legally Kei could not give consent (even to the kiss) but it ends there, LEGALLY. from 16-17 to 18 you literally change nothing apart from being legally an adult, I knew the same wen I was 16, same for 17, 18 going trice and at 19, yep, still the same. My point is, the whole "is a kid" argument is only important wen we are talking about legality of actions, this does not affect the way a character thinks, its not like you change your views on stuff wen you turn 18.
In my opinion (and why I think you are still stuck to that weird "this doesn't feel right feeling"), we are simply "brainwashed" into viewing 17yo as kids and 18yo as adults as a way to create this automatic artificial red flag since every country needs to have their own boundary's on how to handle things (hence why there is no universal age on wen you become an adult), wen in reality, in terms of brain, morality and actions (and some times body), there is no diference. I have more examples of this but dont wanna be annoying
So my advice still stands, unless you are going into shota territory (like 15 or below I guess) try to make a blind eye to the whole age gap business (unless thats the main plot point), because, you should be looking at the characters.
BUT, obviously, if the character acts like a 4th grader (not childish, but like a kid), it becomes automatically weird, as obviously, it portraits a feeling that the character is being taken advantage of because of his inexperience and curiosity, but for this specific story, age is a thing that should not really be the prime focus, but only a subplot/plot point.
Since, while Kei is a virgin and a teen, he already liked Meguru even before knowing him and already had come out to his mother for 1 year (I believe) and never shows lack of maturity, but rather, a huge lack of emotional support, and Meguru is not even shown as a proper adult whit life experience (I think that was kinda the point), apart from him having a job (the author never mentioned if Meguru was a good writer, which imo was also another hint at the fact that he isnt living his ideal life ) and a house (which was his mothers), he starts whit unresolved issues whit his mother and along the story even shows that he dislikes his past and how his fears affected his life, which for me made the story much more interesting, as, specially because of the age gap, we get to se Meguru "being a teen" for the first time, all because of Kei, as he even claims that it feels like he is the one going trough puberty (tbh for me puberty is at 13 so Kei going trough puberty seems like a odd way to phrase it but I guess the author went whit it being a process, but not the point), whit Kei at the end helping him coming to terms whit his mother.
So, while I think the age gap was mainly for that tiny bit of drama of "Oh no, I like someone that is still a student, I am a perv !", but it was also subtly used to develop Meguru, since, instead of the cliche "Experienced adult Seme seducing the student Uke whit home problem", it turned the story into "Broken Adult meets Broken Teen and find mutual support and love in each other"
Side note: Sorry for the wall of text.