Movies =/= real thing. As for your witnessing a suicide, when shock sets in you don't say anything. The first thing to happen in a situation like that is the adrenaline kicks in, you feel jumpy, running around, wanting to get help.... then shock could take over and you become calm, shaky, and quiet. I'm not saying you went into shock but seeing someone shoot themselves in the head is not real violence in the way we've come to understand the word. Violence would be witnessing someone else shoot someone in the head, being placed in a situation in which multiple people are fighting, being placed in war, or being placed in a situation in which you need to defend yourself. That would be violence and I don't believe that anything except for being in those exact situations can prepare you for future situations like them.
Also I disagree with the age and maturity idea. Obviously it is more shocking to a child to be placed in a violent situation than it is for a person in his 20's to be placed in the same situation; however, it makes no difference if I am 20 or 40 if I have had no violent experiences in between those two ages.
Media does affect how people react to violence, but it does not prepare them at all. In fact, media never truly shows the violence despite the constant discussion about it. Media is there to assure the public that they are going to be O.K. while also reporting to the public what has happened.