Self-harm is the largest cause of death among 20- to 24-year-olds in the UK, followed by road injuries, the two causes together responsible for 650 deaths in 2013, according to a global report that finds that the health of young people at a critical stage in their development has been neglected for decades.

The Lancet Commission on adolescent health extends the definition of young people going through the physical, emotional, social and cultural change involved in growing to adulthood, to encompass people aged 10 to 24.

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The authors, backed by new global data from the Institute of Health Metrics and Evaluation (IHME) at University of Washington, Seattle, say the changes in young people’s bodies, their fitness and mental development into their 20s set them on the path to good or poor mental and physical health later on. Yet, partly because young people are considered fit and healthy, this has not had the attention it deserves.

Today’s young people should be the healthiest generation ever, because of better healthcare, nutrition, education and smaller families. But troubling issues have emerged, which many societies are not helping them address.

“Global trends include those promoting unhealthy lifestyles and commodities, the crisis of youth unemployment, less family stability, environmental degradation, armed conflict and mass migration, all of which pose major threats to adolescent health and wellbeing,” they write.

Young people are having to deal with these issues at a time when their brains are going through a developmental surge second only to infancy in terms of the changes occurring within neural systems, says the commission, comprising academics and other experts.

Adolescents seek social engagement and interaction with their peers and they have a heightened response to emotion. The “quality, security and stability of social contexts in which younger adolescents are growing up” is important in enabling them to develop skills to deal with what they are seeing and hearing.

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“It is perhaps not surprising that late childhood and early adolescence are often when the first symptoms of most mental disorders emerge,” says the Lancet report.

In the UK, 329 young people aged 20 to 24 died from self-harm in 2013, which was a fall of 47% from 626 in 1990. Road traffic accidents were the leading cause of deaths from 10 to 19, killing 313, but this is 63% lower than the 843 deaths in 1990. However, the mental health of adolescents in the UK and elsewhere in the developed world is a major issue.

Drug use killed 283 15- to 24-year-olds in 2013, which is a 36% increase on 1990. And depression and anxiety disorders were among the top five causes of ill-health for young women in all age groups in both 1990 and 2013.

Depression was the cause of the largest amount of ill health worldwide in 2013 among young people, affecting more than 10% of 10–24 year olds, according to the new data from the IHME .

The fastest-growing risk factor for ill health in young people aged from 10 to 24 years, over the past 23 years, is unsafe sex. Alcohol remains the world’s leading risk factor for ill health in young adults aged 20 to 24, responsible for 7% of the disease burden, followed by drug use accounting for 2.7%.

HIV and Aids, road traffic accidents and drowning caused a quarter of deaths in 10- to 14-year-olds globally in 2013, with diarrhoeal and intestinal infectious diseases, lower respiratory infections and malaria contributing to a further 21% of deaths.

Road traffic accidents (14.2% and 15.6%), self-harm (8.4% and 9.3%), and violence (5.5% and 6.6%) were the leading causes of death for 15- to 19-year-olds and 20- to 24-years-olds respectively. Among women, 17% of 20- to 24-year-olds and 11.5% of 11- to 19-year-olds died as a result of pregnancy or childbirth.

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Greater investment in young people’s mental and physical wellbeing is critical, say the authors. “This generation of young people can transform all our futures,” says the commission’s lead author Prof George Patton of the University of Melbourne, Australia.

“There is no more pressing task in global health than ensuring they have the resources to do so. This means it will be crucial to invest urgently in their health, education, livelihoods, and participation.”

The commission’s recommendations include free secondary school education for all, laws against marriage under the age of 18 and the gathering of more evidence to improve mental health and prevent violence against young people.

In the UK, the Samaritans can be contacted on 116 123. In the US, the National Suicide Prevention Hotline is 1-800-273-8255. In Australia, the crisis support service Lifeline is on 13 11 14

• This article was amended on 10 May 2016. An earlier version referred to Washington University, rather than the University of Washington in Seattle.

