In my neck of the woods, they're usually on the walls. There doesn't appear to be any standard for how high to place them, it's just wherever it happens to fit. Probably fine for most adults, but shorties and children...nope.
I've seen one recently that was attached to the sink, which was better for being able to reach it, but the place where the water dripped was maybe 6 inches below the air output, so water went -everywhere-.
Being tall, I usually have to hunch over to activate airblades placed somewhere south of my waist, which would be viable for anyone over about 4'. Apparently there's not much consensus on how to place those things.