Thread:Bennia Lover/@comment-5624349-20170523061705/@comment-29963364-20170523071646

First... Issei's birthday is April 16.

Second... The Japanese school year starts in April.

Third... High School graduation in Japan is in March.

When Issei met Rias and became a Devil, it was a few months into the school year, meaning Issei had already turned 17.

After Rias' graduation took place in March, which is just one month before Issei's b-day. And we got a 1-2 month time-skip from then to the start of the World Rating Game Tournament, meaning Issei's b-day had passed by then.

Therefore, Issei is now 18.