Middle school and older seems reasonable for schools to discuss current events of that nature. I was in high school when Columbine happened, and I can still remember the words spoken in fear by a few of my teachers who told us that it might happen to us and ventured guesses as to who might be capable of that in our community. If I had been a bit younger (definitely 9 or under), I don't think I would have understood what was real and what was spoken out of fear. 9/11 was very uptsetting to my young cousins, and their elementary school showed fairly graphic news coverage of it.
Dicussing it at an age-appropriate level is one thing, but showing pictures/news coverage that may be distressing to kids is another issue. I've seen my fair sure of ugliness in war, and I wouldn't wish anyone to see what it looks like, even through the news.
