The starting age depends on the movie, and what kind of experience you want them to have. If you want to sit your kids down, show them all 3 movies, have them understand what's going on, and have them remember it, I'd say 7-8 is the right age.

If you're talking about just Star Wars (Episode IV), I really don't think you can be too young for this movie. It's just good, clean fun. FWIW, I was 5 when I saw Star Wars in 1977. I think you need to be about 4 to "get it" though.

For ESB, Luke getting his hand cut off can be very scary. I think my oldest was about 5 or 6 the first time he really saw this, and he was pretty shaken up. When I saw this in theaters in 1980, I remember being scared and tempted to hide behind the seat, and I was 8. I don't think ROTJ is as scary as ESB.

In my house, any one of the original trilogy can be playing at any time. Over the years my kids have been exposed to Star Wars in bits and pieces. I don't think there's any real harm in that approach, as long as you skip through a few scary parts.