Youth aged 13 to 24 accounted for an estimated 26% of all new HIV infections in the United States in 2010.
Most new HIV infections among youth occur among gay and bisexual males; there was a 22% increase in estimated new infections in this group from 2008 to 2010.
Almost 60% of youth with HIV in the United States do not know they are infected....
... Gay, bisexual, and other men who have sex with men* account for most new infections in the age group 13 to 24 ...
...New HIV Infectionsb Among Youth (Aged 13–24 Years)
•In 2010, youth made up 17% of the US population, but accounted for an estimated 26% (12,200) of all new HIV infections (47,500) in the United States.
•In 2010, young gay and bisexual men accounted for an estimated 19% (8,800) of all new HIV infections in the United States and 72% of new HIV infections among youth.
These young men were the only age group that showed a significant increase in estimated new infections—22% from 2008 (7,200) through 2010 (8,800).
http://www.cdc.gov/hiv/risk/age/youth/