Authorities say the youth, whose name was withheld, was16 years old then.
Investigator Chad Langdon testified Friday that the teen was the most remorseful of the 20 defendants.
The girl was assaulted six times in Cleveland, Texas, over a four-month period in 2010.
Although Langdon said the girl told them the youth did not force her, she's not old enough to legally give consent.
The teen reportedly said he did not know she was only 11 and was sickened when he found out.
He was the fourth of six juveniles to receive probation in exchange for testifying against the remaining defendants.