It's a famous movie about spaceships , aliens and lasers; I had parents with no interest in Sci Fi and I still saw it by the time I was 9, as well as playing all the old school SW video games (XWing, Rebel Assault , Tie Fighter).
What always surprised me is that the star wars films have a "U" rating here in the UK (which means suitable for everyone) rather than at least a PG (meaning suitable for older children with parental discretion) considering the amount of violence.