How high is shower waterproofing treatment supposed to be?
When a 2-year-old shower is leaking, something is definitely wrong. Call the builder, and tell him to fix it.

Yes, there's supposed to be waterproofing behind the tiles. The tiles and grout by themselves won't be enough to keep the water out of the wall. I don't know if there are precise rules for how high the waterproofing should go, but I would think at least as high as the shower head. When I built my shower, I went all the way up to the ceiling.