I think if you watch it and it's not too graphic. I wouldn't want them seeing people coming out all messed up and bleeding. Or the people who jumped to their death. That made me cry when I saw it in real time. Was awful. Honestly, if they saw any of the Boston marathon coverage, that was worse! So graphic!
They aren't too young, they are NOT babies. This IS their history. I don't believe in sheltering kids from current events (or past ones) unless they are very young (the Oklahoma City bombing happened on my daughters 5th birthday, I did NOT let her see it, although later, of course, she was aware of it).
At the time of 9/11, our kids were 11, 8, and 6. They saw the news footage. My husbands aunt lives in a building very close to there (was shown on all the footage) and we kept watching to make sure it was ok. Phones weren't working, and as it turns out, she wasn't even at home, she was across the country (no cell phones for her, she's now 85 and still lives there).
I would let my kids watch it if they wanted to..it is part of history and they should be able to learn about it..just be ready to explain to them about things if they have questions or stop the video if they don't want to watch anymore. My oldest would stop watching if it showed any blood or anything. She hates the sight of blood and gets grossed out.