George Lucas said in an interview, April 1977:

Fox hates for me to say this, but Star Wars has always been intended as a young people's movie. While I set the audience for Graffiti at sixteen to eighteen, I set this one at fourteen and maybe even younger than that.

George Lucas in an interview in the New York Times, published on the 12th of September 1976:

It's for young people. 'Graffiti' was for 16-year-olds; this is for 14-year-olds. Young people don't have a fantasy life anymore, not the way we did. All they've got is Kojak and Dirty Harry. There's all these kids running around wanting to be killer cops.

So..

George Lucas: Fourteen

John Dykstra: Twelve