The study also shows a "worrying" 15 per cent of girls and young women aged 7-25 report feeling unhappy most of the time.
This finding comes from the Girls in Scotland 2018 survey which captured the views of over 500 girls in Scotland on what it is like growing up as a girl today.
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A high number of girls aged 13-25 report knowing another girl their age who has experienced depression (77 per cent), anxiety (68 per cent) and self-harm (63 per cent).
The charity's survey also reveals that as girls get older their happiness declines. Just two in five girls aged 7-11 describe themselves as "very happy", falling to just one in five women among those aged 18-25.
From the pressure to do well at school to worrying about their appearance, the demands of everyday life are having real consequences for girls’ wellbeing.
On World Mental Health Day today, the leading charity for girls and young women in Scotland is calling for "a more holistic approach to promoting wellbeing and resilience, rather than viewing mental health in isolation".
Katie Young, a 21-year-old leader in training from the 12th Clydebank Guides, said: “Our Girls in Scotland research shows that girls and young women are becoming unhappier as they get older.
"This is down to a whole host of issues, including the pressure to do well at school, worrying about what they look like and being bullied. This has a serious impact on girls' wellbeing and is holding them back from living their lives to the fullest."
She added: "It’s imperative that we listen to girls and young women and take them seriously when they tell us what action needs to be taken to improve the mental health and wellbeing of girls and young women."
Girlguiding Scotland offers girls the chance to take part in their Think Resilient programme, which gives girls a space to talk about their mental wellbeing and resilience. It aims to help girls aged 10 and up boost their mental wellbeing, find positive ways to deal with day-to-day pressures and encourage them to speak to someone they trust about what’s on their mind. Activities include positive thinking, self-calming techniques and identifying support networks.