Adolescence

The beginning of adolescence is marked by the onset of puberty, which triggers a period of rapid growth and development of the body and brain, including the attainment of sexual and reproductive maturity. In the past 30 years, the onset of puberty has occurred earlier than before in both boys and girls. Timing of puberty is important beyond adolescence as it is associated with adult health outcomes. For example, early onset of puberty is associated with increased risk of type 2 diabetes, obesity, heart disease and some cancers, including breast, endometrial (i.e., the lining of the uterus) and prostate cancers.

Over the past 20 years, research has shown that the extent and significance of the development of neural systems that occurs during adolescence is second only to those that occur in infancy. During this time, there are rapid changes in the cerebral cortex, which is the outermost layer of the brain that controls key cognitive functions such as thinking, learning, memory and problem-solving. Brain development during adolescence can be affected by many environmental hazards. For example, animal studies have shown that exposure to air pollution can disrupt the pruning of synapses and the development of neural circuitry.

Adolescence is a critical time for emotional development. The myriad of changes that occur in the brain and body of an adolescent are concurrent with social changes involving peers, schools, workplaces and communities, which adolescents need to learn to navigate. The onset of puberty drives changes in the brain’s limbic system, which regulates emotions and behaviours. Adolescents learn to identify and regulate emotions and learn how they affect thoughts and behaviours. In early adolescence, self-esteem is often at its lowest point and tends to improve in middle to late adolescence.

Around one half of bone mass is accrued during adolescence, making it a critical window for bone health as reduced peak bone mass is a risk factor for osteoporosis, which can have significant health impacts in older adults. Environmental factors such as poor nutrition (particularly, reduced intake of calcium and vitamin D), lack of weight-bearing activity, and smoking can negatively affect bone mass.

Adolescents have increased energy and nutrient requirements to meet the needs of their rapidly growing bodies. It can be a challenge to meet these needs while maintaining a healthy weight. Maintaining a healthy weight during adolescence is important because obesity during adolescence is associated with an array of medical problems including prediabetes, type 2 diabetes, fatty liver disease, abnormal levels of fats in the blood, polycystic ovarian syndrome, obstructive sleep apnoea and mental health disorders. Obesity during adolescence is also associated with adult health problems including heart disease and death from any cause.

The cardiovascular system continues to develop in adolescence and sets the stage for cardiovascular health in adulthood. For example, high blood pressure in adolescence is a risk factor for high blood pressure and cardiovascular disease in adults. Air pollution has been shown to be associated with increased blood pressure in adolescents. There is also suggestive evidence that early atherosclerosis can begin in adolescence and be affected by exposure to ambient air pollution, which in adults has been shown to be a modifiable risk factor for heart disease.

Lung function continues to mature in adolescence, developing rapidly around ages 11–15 years in both boys and girls. Lung function growth then slows but continues in girls until the late teen years, and in boys until the early twenties. As adolescents transition to adult status, their lung function can affect the likelihood of having respiratory diseases later in life.

Although globally there have been significant reductions in the burden of and death from communicable diseases among children under 5 years of age, the decline in mortality rate has been less pronounced in adolescents. Research that specifically looks at global communicable disease burden in adolescents has historically been lacking. In 2021, a major global report showed that while gastrointestinal infections, lower respiratory tract infections and malaria were major causes of communicable disease in both children and adolescents, TB and HIV emerged as important causes specifically during adolescence.

Adolescence is an important time to lay the groundwork to develop and maintain physical fitness that can affect lifelong health. There is strong evidence that higher levels of cardiorespiratory fitness and healthy body composition in adolescence are associated with better cardiovascular health later in life. Despite the importance of adolescence as a key time to maintain physical fitness, global data suggests that around 80 per cent of adolescents aged 11–17 years do not meet current physical activity guidelines.

Sleep plays an important role in physical and mental health, immune function and performance in school. In adolescents, the processes in the body that control sleep regulation undergo dramatic changes that lead to adolescents having different sleep patterns. Adolescents generally have later sleep-onset time which makes it difficult to fall asleep early in the evening and get up in the morning. Adolescents 10–12 years old are recommended to get 9–12 hours of sleep while older adolescents are recommended to get 8–10 hours of sleep.