While this is possibly true, I think we should not discount the fact that a lot of prescribed reading (in schools) is not age appropriate literature. If I had read 1984 or 'Lord of the Flies' during my schooldays I would not have been able to appreciate it. I read a lot in that period and I loved to read (and still do) but I seldom liked reading what was prescribed as part of the curriculum - with the sole exception of 'Robin Hood'. It was an 'abridged' version and I read it so many times because I was thoroughly entertained by the story.
I liked 1984 at high school. I loved it, my favorite book at the time.
I agree that Robin Hood and adventure should be assigned soon. The biggest risk with Robin Hood and similar books is that you get it assigned too late, when you cant appreciate story as genuinly as young people.