A fifth of the world's population — 1.2 billion people — is between 10 and 19, more than ever before, the United Nations says. Half are poor and a quarter live in extreme poverty on less than a dollar a day.
"How well we prepare them to face adult challenges in a fast-changing world will shape humanity's common future," the United Nations Population Fund said on its Web site in a prelude to Wednesday's publication in London of its annual State of the World's Population report.
Helping young people with reproductive health issues has become an urgent priority, the report says, calling for more investment in youth-friendly services, family planning and education programs.
HIV/AIDs has become a disease of the young, fueled by poverty and severe lack of information and prevention services, the report says. Half of all new HIV infections are among people aged 15 to 24, and a majority of the young don't know how the disease is transmitted.
In sub-Saharan Africa, where the spread of HIV/AIDS among youths is fastest, about 8.6 million have HIV/AIDS — 67 percent of them female.
South Asia follows, with 1.1 millions youths infected — 62 percent of them female.
Poverty is a factor in the spread of HIV, the report says, because some poor girls exchange sex for money for school fees or to help their families, placing them at risk of infection.
Discussing sexual behavior is taboo in many countries, so many young people do not have the information they need to protect themselves. In Somalia, the report says, only 26 percent of adolescent girls have heard of AIDS and only 1 percent know how to protect themselves.
Among the poorest and least-educated populations, early marriage of girls and expectations of early childbearing persist, contributing to high maternal mortality and reducing girls' chances for education.
The report backs up these conclusions with harsh statistics.
Teenage mothers are twice as likely to die in childbirth as women in their 20s, girls under 16 are five times more likely to die than women in their 20s, and 14 million young mothers aged 15-19 give birth each year. About 5 million girls between 15 and 19 undergo unsafe abortion every year, the report says.
"Studies show that money spent to delay births to adolescents and prevent HIV infections is repaid many times over in direct savings and indirect economic gains," the report says.
"Enabling young people to delay or avoid pregnancy, coupled with education and job opportunities and respect for their rights, can have enormous economic benefits for families, communities and nations and help alleviate poverty at all levels," it says.