These age gap manga are sometimes a bit weird lol
I've seen others were the age gap is big but it's more like 20 year old with a 30 year old. It still keeps the interesting age gap problems while having both be adults. But these kind where it's an old adult with a child are a bit peculiar
It would be wayyy more "reasonable" if the kid was at least 17 or 18 - he would still be in high school and the manga could retain that "age gap" storyline but he would also old enough to make his own decisions and think for himself
At a younger age like 15, even though the manga may seem cute, if it were in real life it would basically be considered really strange and looked down upon. It's basically an adult with a child.
If you still don't think it's a bit weird, just imagine yourself at age 27, already graduated from college with a full-time job. Then one day you start talking to a 15 year old kid who's still learning basic algebra. And then you try to start a relationship. Really?
There's another age gap manga where the lead is an older woman and the male character is a kid. But the manga treats it way more realistically. She treats him more like a son and gives him the support he needs, rather than being a normal love relationship.