"You turn on the television, and violence is there. You go to a movie, and violence is there." 1
Psychologist Craig Anderseon, PhD, DIrector of the Center for the study of Violence at Iowa State University, concluded that the "evidence strongly suggests that exposure to violent video games is a casual rish factor for increased aggressive behavior, aggressive cognition, and agressive affect and for decreased empathy and prosocial behavior."
Again, this particular study focused on adolescents between the ages of 10-14 years old. There was no evidence of long term effects among normal growing adolescents. Some long term risks depended on the environment in which the child is raised in. If they are constantly watching violent media and it plays out in their reality, they are more likely to grow up more aggressive.
If the teen has a mental illness, the aggression plays out more into the real world. Their brain is less likely to tell the difference between the two.