[zzhk]

The problem is once Ise starts accepting servants, Rias' group isn't going to stand idle. They will start fighting for his pieces instead.

It seems to have been retconned, but originally Ise thought Rias would start official Rating Games one year later when she graduates high school. Later on, he thinks she will start after finishing university.

Anyway, if Rias is too young, then wouldn't Ise be even younger?
So who knows what the author actually plans to do.