The present generation of people aged 10–24 years is the largest in history—with a population of 1·8 billion,1 they comprise a quarter of the world's population. Nearly 90% live in low-income and middle-income countries where they constitute a far greater proportion of the population than in high-income countries because of higher fertility rates (figure 1). The growth in adolescent populations coincides with a reduction in infectious disease, malnutrition, and mortality in infancy and early childhood, shifting attention to sexual and reproductive health, substance misuse, mental health, injury, obesity, and chronic physical illness, which become prominent during adolescence and need very different responses.3, 4
Many countries have entered a demographic transition in which falling fertility and longer, healthier life expectancy increase the proportion of people able to work. A healthy, educated workforce has the potential to shape a country's economic prospects.5 Conversely, poverty, inadequate education, mass unemployment, migration, natural disasters, and war result in social environments that can devastate the health of young people.6, 7, 8, 9, 10, 11
Young people were at the forefront of the social unrest across north Africa and the Middle East that began in Tunisia in December, 2010. Although many succeeded in toppling the restrictive regimes that they fought against, they faced serious threats to their lives and health. Such engagement is a powerful reminder that, by stark contrast with younger children, adolescents have an increasing capacity to be active agents of change within their communities.11, 12 It raises concerns about the extent of young people's exposure to violence, exploitation, and abuse, and suggests the need for greater protection of human rights.13
Many of the economic, educational, and political issues that affect young people are interlinked. Investment in education of adolescents has clear benefits to individuals and their health, but is also a strategy for enhancing employment, human rights, social capital, and community wealth.5 The adverse effects of child marriage and pregnancy at a young age (<18 years) on the health and human rights of girls is well appreciated, but just as potent is the dislocating effect of early pregnancy on girls' education, skill development, and social networks, which all undermine their present and future health and wellbeing, the health of their children, and their nations' social and economic prospects.14, 15
Key messages
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At 1·8 billion, young people aged 10–24 years comprise more than a quarter of the world's population.
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The health of adolescents has improved far less than that of younger children over the past 50 years.
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The present generation of young people will take a different path through adolescence from previous generations and will face new challenges to their health and wellbeing. How they negotiate these years will have a powerful effect on their future health and their countries' economic and social prospects.
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Biological aspects of puberty mark the start of adolescence, and key social-role transitions have historically signalled the end, which is now less distinct than in the past. Neurocognitive development is another element of biological maturation with major effects on decision making, emotional wellbeing, and behaviour.
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A life-course perspective emphasises that the health of adolescents is affected by early childhood development and the biological and social-role changes that accompany puberty, shaped by social determinants of health that affect the uptake of health-related behaviours. The onset of these behaviours and states in adolescence affect the burden of disease in adults and the health and development of their children.
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Social determinants that negatively contribute to adolescent health are structural (such as poverty and sex inequality) and proximal (such as intrafamilial violence, parental mental disorder, and substance misuse). These determinants often cluster within individuals. Protective effects are important targets for preventive interventions.
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The ubiquitous nature of social media has changed the speed at which sociocultural norms are affected, with both risks and opportunities.
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Adolescents can be powerful agents of personal change and community development.
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Adolescents should be more prominent within future global public health policies and programming.
Societies typically define adolescence in terms of age and social roles with little consistency between countries. We focus on adolescents and young adults aged 10–24 years (referred to as young people and youth and hereafter referred to as adolescents) because this age-group encompasses most individuals who are going through the biological changes and social-role transitions that historically defined adolescence (panel). Although the biological sequences of puberty are highly consistent (table 1), changes in the timing of puberty, the nature of social-role changes, and the hopes and aspirations of adolescents across the world are widely affected by economic and sociocultural factors.
We assess the role of adolescence as a foundation for future health, emphasising the changing context of health and social development from late childhood to early adulthood and the place of adolescents within global public health. Our report is arranged around a conceptual framework that we have developed to describe the many factors affecting adolescent health and to put into context the subsequent reports in this Series (figure 2).