My son is almost 6 and has seen all the other films. My Wife is supposed to take him tomorrow morning and I saw it last night myself.
I'm thinking he would be fine for it having seen the others as well as being in to the Avengers films and Civil War.
There definitely was a lot of stuff that happened, but there was also the point that no one really not stabbed through like in TFA, no dismemberment, and very minimal blood.
It's so hard trying to remember what I was watching at 6 years old, but I do remember seeing such tragedy as Neverending Story with Artex at 6. My Dad took me to see Empire Strikes back when I was 2 and I loved Star Wars after that.
Thinking back to being a kid, my thoughts were not as deep as adults tend to get. I just remember there was bad guys and good guys and that it was the battle against the 2. There was no deep understanding of actions and theories.... it was just pretty black and white to me.
As the years have gone on I have loved continuing to watch the films and develop my relationship with the films on a deeper level and to this day ask questions on things I have never thought of. I think as adults and now parents (if applicable) we panic thinking what some of these things will mean to our kids, when sometimes we're the problem and can overthink everything. I think that's the debacle I'm in right now. My son does understand things in movies are not real, but some images can still make for a powerful impact regardless. In my reflection of a kid growing up I just don't recall ever Star Wars affecting me past being the coolest thing ever with the most interesting characters and imaginatively rich-filled worlds and creatures I've ever seen and believed in.
I think its a hard question to simply answer to anyone questioning whether or not to bring their kids to see any film. Even some of the most G-Rated films can contain some awful tragedy to scar a child, depending on what it is.
