I think 5 years old is a good age to start, I was that age when I saw the original in '77. I have twin 5 year old nephews and they love SW, absolutely love it. But right now they like the action, and the droids. They love R2-D2, especially in the original when the Jawas shoot him and he falls straight down, they like to mimic that on the floor, by falling over.
When they watch the PT, and the romance, they kinda giggle, and think its icky, being that their young boys. But then again, I had the same reaction of Padme/Anakin when I saw it in 2002 at the age of 29, so go figure!
I really think they should watch the original first, cause that is the funnest movie of the 6. They'll love the characters, be scared of Vader, and enjoy the beginning with the droids walking on Tatooine. I guess everyone here should mimic their first experience in SW, because the fact that were all still talking about it, atleast the OT, means if it ain't broke don't fix it.