[JURIST] The British prisoners rights group Reprieve [advocacy website] has claimed that over 60 of the foreign detainees imprisoned by the US at Guantanamo Bay [JURIST news archive] were juveniles under 18 years of age when captured, and that some were as young as 14. Reprieve told the Independent newspaper that 17 detainees on the latest US list [JURIST report] were under eighteen when seized, and that another seven were probably juveniles. Reprieve founder Clive Stafford Smith [Wikipedia profile], a British lawyer acting for several Guantanamo detainees who has previously pressed the US on the status of children at the camp [2005 article], added that according to information from inmates and the Red Cross, another 37 inmates were under 18 when they were taken into custody.
The US says that there are currently no juveniles held at the prison. Three Afghan youths who were at one point held there in a separate facility called Camp Iguana [Wikipedia backgrounder] were released in 2004 [JURIST report]. The Independent has more.