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Modern youth may roll their eyes at the statement, but researchers recently confirmed its truth: being an adolescent today is a whole lot different than it was years ago.

The adolescent phase in human development now lasts much longer than it once did, leading to a potential rise in its characteristic risks, including sexually-transmitted disease and excessive drinking, Susan Sawyer, MD, of the Murdoch Children's Research Institute in Melbourne, Australia, and colleagues reported in a special issue of The Lancet dedicated to adolescent health.

"In contrast to older views that saw adolescence as a brief period of turmoil, of little significance to future health, we're suggesting in this paper that we need to appreciate that adolescence is a resource for future health and national wealth," Sawyer said during a press briefing on the special issue.

Today, the phase of adolescence begins at age 10 and ends at age 24, expanded at both ends by an earlier onset of puberty and a delay of mature social roles, respectively.

Despite its "widespread legal significance," Sawyer and colleagues wrote, "the age of 18 years clearly no longer signifies adulthood in many parts of the world."

The majority (90%) of the world's 1.8 billion adolescents live in low- and middle-income countries, due to falling fertility rates in high-income countries, the researchers said.

The group is so numerous because of a reduction in infectious disease, malnutrition, and death in infancy and early childhood. While infant and early childhood mortality has fallen 80% over the last 50 years, adolescent mortality has only marginally improved, the researchers said.

Most deaths in this age group are due to injury, such as car accidents, which are largely preventable, Sawyer and colleagues wrote.

Changes in the biological and social factors that define adolescence have also influenced health in this population. Both the earlier onset of puberty and the later entrance into social norms, such as marriage, extends the period during which adolescents are vulnerable to sexually transmitted infections and unplanned pregnancies.

The "heightened sensitivity" to peer pressure during this extended phase can lead to a greater likelihood of substance misuse and abuse, the researchers wrote, while more targeted marketing of unhealthy products and lifestyles -- particularly by the tobacco industry -- also has an impact.

And although greater exposure to social media can encourage engagement among young people, it can also lead to problems, such as decreased physical activity, sleep disturbance, and cyberbullying, they said.

That's why Sawyer and colleagues called for greater attention to adolescent health on a global level, in order to "ensure that adolescence is indeed a strong foundation for future health."

Organizations should develop agendas that "place the developmental phase of adolescence center stage, rather than on any one health issue," they wrote, and they should promote greater involvement of adolescents in their own health choices and decisions.

The researchers also called for greater funding of adolescent health programs, including academic institutes and specialized undergraduate and postgraduate programs.