The average height for a shower head is 6 feet and 8 inches from the bottom of the floor which is the minimum height required by code and industry standard for shower head installations.
Of course you can place the shower head up a little higher on the wall if you want too but you cannot really go lower than the 6 feet 8 inches if you're having the work inspected.
But if you're not having the work inspected by an inspector then you can put it at any height that is suitable for you and your family.
But I would place the shower head at least 6 feet 8 inches which is standard.