User blog comment:Bennia Lover/Age of Characters/@comment-177.34.193.20-20170509044514

Just leaving my thoughts here. iirc the year children in Japan enter elementary is determined by whether or not they were born on or before April 1. Children born after this date have to wait until the next year. Issei was born after that date so he had to wait. Going by this logic, not all characters in the same grade were born in the same year.

In his second year, Issei was 17 but his classmates that already had their birthdays before or on April 1 were 16. Rias and Akeno were 18 but part of their class were 17.