Actually the data is very conclusive that exposure to violent movies/games raises levels of aggression (in young adults as well, not just children). It just isn't widely believed.

It has been shown that if children watch the violent movie/tv with the parents than there is very little effect. Which goes back to the point drone was making.



Media violence and aggression. Effects of media violence on aggression for different types of studies. Diamond widths are proportional to the number of independent samples. There were 46 longitudinal samples involving 4975 participants, 86 cross-sectional samples involving 37,341 participants, 28 field experiment samples involving 1976 participants, and 124 laboratory experiment samples involving 7305 participants. Red lines indicate the mean effect sizes. Blue lines indicate a 95% confidence interval. Note that zero (dashed line, indicating no effect) is excluded from all confidence intervals.
A positive link between media violence and aggression regardless of research method is clearly shown (see the figure). Experimental studies demonstrate a causal link. Laboratory experiments yield slightly larger effects than other studies, presumably because of greater control over irrelevant factors (see the figure). Field experiments demonstrate causal effects in naturalistic settings. Cross-sectional studies demonstrate a positive association between media violence and types of real-world aggression (for example, assault) that cannot be studied ethically in experimental settings. Longitudinal studies reveal long-term effects of early media violence exposure on later aggressive acts. These effects are not trivial in magnitude. For example, they are larger than the effects of calcium intake on bone mass or of lead exposure on IQ in children (4). Interestingly, recent work demonstrates similar-sized effects of violent video games on aggression
six major professional societies in the United States--the American Psychological Association, the American Academy of Pediatrics, the American Academy of Child and Adolescent Psychiatry, the American Medical Association, the American Academy of Family Physicians, and the American Psychiatric Association--recently concluded that "the data point overwhelmingly to a causal connection between media violence and aggressive behavior in some children"
Despite the consensus among experts, lay people do not seem to be getting the message from the popular press that media violence contributes to a more violent society. We recently demonstrated that even as the scientific evidence linking media violence to aggression has accumulated, news reports about the effects of media violence have shifted to weaker statements, implying that there is little evidence for such effects (4). This inaccurate reporting in the popular press may account for continuing controversy long after the debate should have been over, much as the cigarette smoking/cancer controversy persisted long after the scientific community knew that smoking causes cancer.