I always see this comment about age gap whenever the protagonist has more than 3 years age gap.
1. Age gap is not rare in real world.
2. Why he is 20 y.o something related with his lifestyle before he become high school student and he has reason why he want to become high school student. It's interesting for me and make me wonder what kind of lifestyle he had so he couldn't go to highschool before 20 y.o.
3. 2nd highschooler is 16 y.o and it's marriagable age for woman in Japan.
I am always wondering why people always complain about age gap as if it's a sin when there are a lot of people with huge age gap like president of France for example. If she is still in elementary school or middle school then sure, complain, but 16 y.o. is marriagable age.