I was about two years old when I first saw Star Wars. Obviously, I have no memory of "first seeing it." I obviously understood little as well, I am told I laughed my head off at the cantina scene though. But it's kind of nice having something always there in your life, since before a time you actually have memories of.
So I don't think there is any "correct" age to show a kid the film. I say show it them as early as possible, worked for me after all.