I assume it's a senior year thing, so 21 and 18 really aren't that different, everyone always gets hung up on these things, not realizing that all the age laws we have now, really haven't been around that long, like a little over 100 years or so, my parents had a 9 year difference between them, and two adults are two adults, period.
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what an interesting thing to say 🤔 wondering how old ur parents were when they started dating + theres a difference between 12 + 21(JAIL!!) and 50 + 59(comepletely normal and okay). i also dont agree with the age laws you're probably talking about (romeo and juliet)... i dont have a problem with age gaps as long as they're reasonable which 18 + 21 (which is not even confirmed if hes 18) is weird to A LOT of people including me hence my comment and also not knowing whether (yui/yuri? dont remember name) is 18 or younger.
Ok, first off, 12 and 21 is impossible in high school, that's generally grades 9-12 or 10-12, so that part of your comment is completely invalid. And it may be the authors fault for not putting a label of all characters are 18+ at the beginning. But honestly, it's just fiction, so if your sensibilities hurt from it, why can't you simply change the story to him being 19 instead.