KABUL, May 26 (Reuters) - An Afghan who has spent over six years at the U.S. military's Guantanamo Bay prison was only around 12-years-old when he was detained, not 16 or 17 as his official record says, an Afghan rights group said on Tuesday.

Interviews with the family of Mohammed Jawad, who like many poor Afghans does not know his exact age or birthday, showed he was probably not even a teenager when he was arrested in 2002, the Afghan Independent Human Rights Commission said.

He was picked up by Afghan police in connection with a grenade attack in Kabul in which two U.S. soldiers and their Afghan interpreter were wounded. He was transferred to U.S. custody the same day and flown to Guantanamo in early 2003.