Girl guides' leaders are warning that girls face sexual and consumer pressures to grow up too soon and many see self-harm as "normal behaviour".
Girlguiding UK says that these pressures of "premature sexualisation" and materialism can lead to bullying.
Its warning is based on the results of an online survey of girl guides aged between 10 and 14.
Chief Guide Liz Burnley warned that too many girls now experienced "stress, anxiety and unhappiness".
A report from Girlguiding UK, A Generation Under Stress?, published in partnership with the Mental Health Foundation, reveals a generation who feel they are being forced to grow up too quickly.
The research suggests an experience of childhood very different from the traditional guiding image.
It claims that girls feel under pressure to wear clothes that make them look older and have to cope with advertising and magazines selling adult images.
As well as feeling under pressure over their weight, young teenagers are pushed by websites and magazines to consider plastic surgery, Girlguiding UK reports.
Two-fifths of the girls taking part in the survey said they felt worse about themselves after looking at magazine pictures of models, pop stars or actresses.
There were other signs of the perceived anxieties facing this age group. Two in five knew someone who had self-harmed, two in five knew someone who had panic attacks and one in three knew someone with an eating disorder.
The girls expressed their fears about needing to be seen with fashionable clothes and gadgets - making them feel bad about themselves and making them feel vulnerable to bullies if they were unable to keep up with their peer group.
The most important protection against the anxiety from this was to have "stable and supportive families and friendship groups".
"Young girls today face a new generation of pressures that leave too many suffering stress, anxiety and unhappiness," said Chief Guide Liz Burnley.
"All of us who care about young women have a part to play in helping them find a way through these conflicting demands to build the confidence they need to be themselves."