It seems that society and the common legal definition have got it about right: it takes some 21 years for a human being to reach adulthood. The evidence shows a particular need to invest in the crucial development period from conception to age two (the first 1,000 days) and also during critical phases over the next 7,000 days. Just as babies are not merely small people—they need special and different types of care from the rest of us—so growing children and adolescents are not merely short adults; they, too, have critical phases of development that need specific interventions. Ensuring that life’s journey begins right is essential, but it is now clear that we also need support to guide our development up to our 21st birthday if everyone is to have the opportunity to realize their potential. Our thesis is that research and action on child health and development should evolve from a narrow emphasis on the first 1,000 days to holistic concern over the first 8,000 days; from an age-siloed approach to an approach that embraces the needs across the life cycle.

To begin researching and encouraging action, this volume, Child and Adolescent Health and Development, explores the health and development needs of the 5 to 21 year age group and presents evidence for a package of investments to address priority health needs, expanding on other recent work in this area, such as the Lancet Commission on Adolescent Health and Wellbeing (Patton, Sawyer, and others 2016). Given new evidence on the strong connection between a child’s education and health, we argue that modest investments in the health of this age group are essential to attain the maximum benefit from investments in schooling for this age group, such as those proposed by the recent International Commission on Financing Global Education Opportunity (2016). This volume shares contributors to both commissions and complements an earlier volume, Reproductive, Maternal, Newborn, and Child Health, which focuses on health in the group of children under age 5 years.

There is a surprising lack of consistency in the language used to describe the phases of childhood, perhaps reflecting the historically narrow focus on the early years. The neglect of children ages 5 to 9 years in particular is reflected in the absence of a commonly reflected name for this age group. Figure 1.1 illustrates the nomenclature used in this volume, which we have sought to align with the definitions and use outlined in the 2016 Lancet Commission on Adolescent Health and Wellbeing. The editors of this volume built upon the commission’s definitions to include additional terms that are relevant to the broader age range considered here, including middle childhood to reflect the age range between 5 and 9 years. The editors also refer to children and adolescents between ages 5 and 14 years as “school-age,” since in low- and lower-middle-income countries these are the majority of children in primary school, owing to high levels of grade repetition, late entry to school, and drop outs. As income levels rise and secondary schooling enrollment increases, children attending school will be older than age 14 years. Figure 1.1 also demonstrates the overlap between many of these terms. For example, the Convention on the Rights of the Child defines child as every human being younger than age 18 years, whereas this volume defines adolescence as beginning at age 10 years and continuing through age 19 years (United Nations General Assembly 1989). Figure 1.1 also shows the alignment between age groups and four key phases critical to development. These key phases are used as an organizing principle for intervention throughout this volume. Where possible, the editors have extended the analyses to include children through age 21 years; but standard reporting of age data is in quintiles, so for convenience the editors have accepted the upper age range as 15-19 years.

Some issues of potential importance to child development are examined in other volumes of DCP3. For example, environmental issues are examined in some depth in volume 7 (Mock and others 2017), which examines the impact of pollution on health and human development—especially the exceptional prevalence of lead poisoning, which affects the intellectual development of children.

A premise of this volume is that human development occurs intensively throughout the first two decades of life (figure 1.1), and that for a person to achieve his or her full potential, age- and condition-specific interventions are needed throughout this 8,000 days (box 1.3). We use four key tools—cost-effectiveness, extended cost-effectiveness, benefit-cost, and returns on investment—to identify and prioritize investments at different ages and to propose delivery platforms and essential packages that are costed, scalable, and relevant to low-resource settings. These analyses suggest that public investment in health and development after age 5 years has been insufficient. Investment lags far behind the potential for return and is far below investments in health in the first five years and in primary education after age 5 years. Table 1.1 compares our recommendations for additional spending with current spending on education and with spending on health for children under age 5 years.

This bias in investment is paralleled by a similar bias in research. Approximately 99 percent of publications in Google Scholar and 95 percent in PubMed on the first 20 years of life focus on children under age 5 (annex 1A shows the number of publications since 2004 that our search found that include the terms health, mortality, or cause of death). The availability of age-specific publications reflects a lack of research funding for and attention to middle childhood and adolescence, resulting in a lack of data. The analysis for the Global Burden of Disease 2013 came to a similar conclusion, pointing out that most of the unique data sources for risk factors for adolescents ages 15–19 years were from school-based surveys, that children younger than age 5 had the most data available of any age group, and that adolescents ages 10–14 years had the fewest data sources (Mokdad and others 2016). The World Development Report 2007: Development and the Next Generation similarly found severe data shortcomings for these older age groups (World Bank 2006), whereas Hill and others found no empirical studies of mortality rates for the age group 5–14 years in countries without vital statistics, which include the majority of low- and middle-income countries (LIMCs) (Hill, Zimmerman, and Jamison 2017). The estimates, based on Demographic and Health Surveys Program data, reported here result in sharp upward adjustments in estimated numbers of deaths in that age range (Hill, Zimmerman, and Jamison 2017). This strong bias toward early childhood in the health literature may have been helpful in the successful United Nations Millennium Development Goals (MDG) drive to reduce under-five mortality. But it seems to have caused us to lose sight of the fact that the subsequent decades of growth and development in the transition to adulthood involve complex processes and critical periods that are sensitive to intervention.

This volume focuses on the scientific evidence, but local contexts, including culture, beliefs, lifestyles, and health systems, as well as other key determinants such as gender, race, ethnicity, sexuality, geography, socioeconomic status, and disability, are important for developing practical policies (Chandra-Mouli, Lane, and Wong 2015). Some groups that tend to be marginalized and overlooked when planning intervention strategies, such as ethnic minorities, LGBT (lesbian, gay, bisexual, or transgender) youth, persons with disabilities, youth in conflict areas, and refugees, are also likely to have the greatest need for health and development support.