User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-25792629-20150531215445/@comment-16281343-20150603052838

Yes. And if you map out their ages.


 * Issei should be 13 years old and on his 1st year of junior high.

Tobio describes Vali on -SLASHDOG- as someone who appears to be on the upper grade of elementary school; which put his age around 11-12


 * Issei describes Vali when he first met him as a person who is around his age but younger.

So, all in all. The math checks out.