User blog comment:B214/Volume 22 spoilers/@comment-73.46.6.122-20161126201526/@comment-10330822-20161129200056

I'd be more inclined to believe that if Issei was the same age as Rias, but he still has another year at Kuoh Academy. Granted Rias was going to be married in her 3rd year, but still. Also, Goku and Chi-Chi getting married was an end of the series sort of thing, the series is continuing one probably after this tournament, I doubt its going to all that's left.