For anyone confused at Minato and Mikoto's age.
Mikoto is older, but he was born in April.
Minato is younger but was born in March.
They are not twins, it just might be a nickname or smth their classmates call them since they're int the same grade.
The reason that they are in the same grade despite Mikoto being older is because Japan's school system is weird.
School grades are determined by a child’s age.
Children born between April 2 to April 1 of the following year will be in the same school grade, since the Japanese school year begins in April and ends in March.