Young girls see and hear so much more these days then we ever did. They are still immature but overloaded with info they really do not need.
When my daughter was 14 it was a fashionable 'thing 'in her year for girls to be stick thin. Since she was already very petite it did not take much for her to become underweight, to the point where her teacher contacted me and said she was worried by my daughter's appearance.
I took her to my doctor for something else, but had mentioned it quietly to the doctor before we went in... who told her that her BMI was 16, and she openly asked her do you like looking ill ??
It shocked her that someone had openly said something negative since I had not.
I was afraid she was becoming anorexic and did not know how to deal with it, but the doctor's comments rude as they were made her snap out of it. I always think we were on the edge of something really bad and managed to escape it.
I think the most damage can be done by thoughtless remarks made with good intentions by family. It can make the child do exactly the opposite out of sheer damnedness and then it can all become too late.
In my daughter's year there were some girls who were anorexic, and seeing this also helped her to realise it is no picnic being thin.
Hope that makes sense.