Since everyone else has already gone so much further into the details of the case than any of us ever wanted to read, I will just make one small contribution to the discussion. I'd like to provide some perspective on relative ages of teenagers and their sexual maturity that I think is sorely missing in this hyper-sexualized and sexually schizophrenic culture of ours.
I had the pleasure, if that is the word, of teaching reading classes to kids of all ages a couple of summers ago. I feel like I taught a healthy mix of urban and rural, rich and poor, white and minority children. Among those kids were somewhere around 100 junior high students and and the same number of high schoolers. New teachers are usually stricken by how young the high school kids look, as I was. The junior high kids, the 12-14 year olds, looked like awkward, oily children. The most "matured" among them could have passed for young, late-blooming high school juniors, but would have looked like little kids on any "high school" TV show.
Think about it: for most of us, the images we have of adolescents come from television, where the "high school age" characters are overwhelmingly played by people in the early 20's. 18 year olds look younger than you probably think. A 13 year old who "looks like she's 18" still looks very much like a kid.
For example, remember Freaks and Geeks? Linda Cardellini (Lindsay Weir) was 24 at the time. Natasha Melnick, who played Cindy Sanders (Sam Weir's love interest) was 15.
And don't forget, Polanski's crime was in 1970, and girls apparently mature a lot younger nowadays.
My point is that the girl he raped was much more a little girl than most people are probably imagining. When Roman Polanski and his apologists try to make his sexual proclivities look more normal by saying "she looked 18," she probably looked less like the lead actress in High School Musical and more like Winnie Cooper, season one.