This movie is probably fine for most 5-year-olds, though we sent our daughter out of the room and fast forwarded through the part where Luke returns to find his aunt and uncle murdered and their farm burned. My daughter loved it at age 5, as well as episodes V and VI (though she had a few criticisms of those, mostly character flaws she didn't like).
I'm surprised at the reviews that say other little kids found it boring or confusing, but that's why you have to know your kids. I did have to explain what a smuggler was, and a bounty hunter, but the rest made sense to her.
As long as you watch with them, and are ready to turn it off if it isn't well received, I see no problem with giving this movie a try with a 5-year-old.