Star Wars-- the original, real Star Wars, the one that was released in the 1970s-- is probably fine for any kid who's old enough to watch and understand movies. I saw it when it was first released, at age 4 or thereabouts. There were some parts I found scary (the Sand People sequence was pretty intense), but it was fine. Empire is darker, of course, and the plot is a little more complex, so I think a little kid might have a hard time understanding it; I'd think a six year old (?) could handle it, but if a parent is there with him to explain stuff, I don't think younger kids would be scarred by it. Return of the Jedi is just silly fun; I imagine it would be fine for little kids.
I can't really speak to the other trilogy. As a child, Star Wars was at the center of my imagination. I lived and breathed it and couldn't wait until the new movies were released (I remember that they were originally slated for 1991; an eternity!) Then I saw the Phantom Menace... and it was so, so awful.
Worse, it was awful in such a way (juvenile, stupid) that it almost ruined the original movies for me: it made me wonder whether I liked them because they were any good, or because I saw them at an impressionable age. Watching them again, I can say that the movies are okay, but they really are kids' movies; I can enjoy them now, but that enjoyment is inseparable from nostalgia. I don't think I'd like them if I saw them for the first time today. And I won't watch episodes 2 or 3, because I'm pretty sure they'd kill my love of Star Wars forever.