It's an old movie, I think from 70's.
Main character is a blonde beautiful young woman (adolescent), living with her dad, and ends up having an affair with dad's friend.
She is kind of promiscuous girl, but unhappy although she seems careless.
It's american movie I think, and she was at the age of 16 at the time of making the movie.
I think she received an Oscar for the role, or was nominated.
On the movie poster is she in a dress on a bike.
Can't remember that film's name? Post the plot, and see if other users can help.