What's their age. I mean she looks 11 and he looks 20. I can not understand.

im not sure if im right but 12 and 15

It just look too mature, maybe the artist want to give us an eyecandy by give it image of adult person
They just got 2 years gap

*Their ages*
Lippe: 12 (with an adult's mind)
Rias, Robel and Hildert: 15
Lus and Hal: 19