Horrifying subject!!! Parents must take initiative for this if your ward is using excessive smartphone just take away it from him. He might throw tantrums but, try to avoid them. If you are in such condition where children is grown up visit the psychologist. Smartphone is not just harmful for eyes but also can effect their mental health.
My opinion, first parents carelessly give smartphone to their children so they don't disturb them while they are working, talking, using smartphone, etc possible reasons. In results such condition occurs.