There is actually a boy present in this picture, but let's just ignore him and focus on the girl. This is a head-study from 1716, showing a French girl who is probably not that old but a teenager (though that is a termed that would have meant nothing at the time).
The female hair-dos of the rococo are very, very flat - an interesting contrast to the rather voluminous dresses of the time. The hair was not cut short, but it was not that long either. It was kept flat on the head, not a lot of curls and waves, just very simple. The back of the head was often adorned in some way, with a ribbon, a little cap or something else that was light and frilly - in this drawing it is actually quite hard to see what that would be. It is obviously a study of the same girl (with the same coiffure) from different angles, but the back is kept somewhat sketchy. The focus is obviously the face and how that is perceived and the hair is just a frame to the face with no particular interest in itself. Considering how the hair was generally shaped at the time it is easy to imagine that was not just the view of artists studying young girls.
The boy is interesting as a contrast, he actually seems to have more hair than the girl - something that is not very common in era, except this one (well, you might include the baroque in that statement too).