The number of adolescents infected with HIV has jumped by one-third over the past decade, according to the World Health Organisation.
"More than two million adolescents between the ages of 10 and 19 years are living with HIV," marking a 33 per cent rise since 2001, the WHO said.
"Many do not receive the care and support that they need to stay in good health and prevent transmission. In addition, millions more adolescents are at risk of infection."
The big rise is most marked in sub-Saharan Africa where many born with the virus are now adolescents.
Girls there are the worst affected, with many having unprotected sex.
In Asia the most vulnerable groups are drug users.
"Adolescent girls, young men who have sex with men, those who inject drugs or are subject to sexual coercion and abuse are at highest risk," said Craig McClure, head of HIV programs at the UN children's agency UNICEF.
"They face many barriers, including harsh laws, inequalities, stigma and discrimination which prevent them from accessing services that could test, prevent, and treat HIV.
"About one-seventh of all new HIV infections occur during adolescence. Unless the barriers are removed, the dream of an AIDS-free generation will never be realised."
The increase in infections is reflected by an increase in AIDS-related deaths among adolescents, with 70,000 adolescents dying in 2005, but more than 100,000 fatalities in 2012.
That is in stark contrast to a 30 per cent decline in deaths during the same period in the general population.
"Adolescents face difficult and often confusing emotional and social pressures as they grow from children into adults," said Gottfried Hirnschall, head of the WHO's HIV/AIDS department.
"Adolescents need health services and support, tailored to their needs. They are less likely than adults to be tested for HIV and often need more support than adults to help them maintain care and to stick to treatment."
Among the measures needed, the WHO said, is an end to the requirement for parental permission to have an HIV test.
In sub-Saharan Africa, it is estimated that in the 15-24 age bracket, only 10 per cent of young men and 15 per cent of young women know their HIV status.
In other regions, although data are scarce, access to HIV testing and counselling by vulnerable adolescents is consistently reported as being very low, the WHO said.