Huh? It does not take any rigorous science to understand that stimulus we process has 'some' effect on us.
I'm not claiming playing FPS is harmful (we'd need some science to prove that).
I am claiming that it most likely has some effect, being good, bad or indifferent.
But to claim that since children can tell the difference there is no issue, is, short sighted.
My point is that it doesn't come down to understanding the difference between reality and fantasy, there is more at play here than simply understanding the difference.
Example 1: I know that most commercial porn is fantasy (staged, porn actors, etc). Yet viewing this porn is likely to have some influence on my arousal, despite me knowing that those three ladies are not going to let me do that to them!
Example 2: I watch a high impact and high suspense movie. I know this is total fantasy, yet I still feel emotions - excitement, anger, sorrow, etc.
Example 3: A child watches a horror movie, knows very well that it's just fiction, yet they feel fear - fear that may have a lasting effect on them (alone in the dark 1 year later and that movie may still effect them).
Just because I understand that something is fantasy has little to nothing to do with how if effects me.