The "18+ + 18- = bad" auto-comments notwithstanding, this is a cute game!
Aside from the visual stylistic choices in representing the characters, where do y'all see an adult in this narrative?
This is a story of a kid (who is out of school) getting pressured by his parent to hang out with another kid (who is still in school), with them gradually growing emotionally attached to each other... Told as if by a kid who may or may not be out of school themselves.
All mostly devoid of the unneeded gritty realism edginess.
If the author wants to avoid this bs controversy more, though, I wonder if ages weren't mentioned at all, people would chill out? The taller kid could have graduated school early (and would naturally consider himself an "adult," despite still being a kind living with his mom), so you could essentially make them both vague "kids around same age." Likewise, easy to change dialog like "I'll be X soon" to "I'll graduate soon, too!," etc.
Specifying ages triggers people too much.
This kind of visual representation itself of their height and perceived age differences is not unheard of (Gravitation comes to mind... anyone here old enough to remember *that*?)