Aaron Kean, who was 10 at the time of the crime, told a judge Thursday that he brutalized the 3-year-old because the boy was bothering him and wanted to use his scooter, people who attended the closed juvenile proceeding told The Star-Ledger of Newark.
Kean pleaded guilty to kidnapping and murder and is expected to be sent out of state to a specialized children's mental health facility.
Under terms of the sentence, he will be evaluated periodically and could be released at any time it is determined that he is rehabilitated. If he is released early, he would be subject to six years of supervision, said Middlesex County Assistant Prosecutor Christopher Kuberiet.
Authorities said Kean lured Amir Beeks from a library in March before beating the boy in a backyard playhouse and leaving him face down in shallow water with his pants down, leading to an initial sexual assault charge. The child died the next day at a hospital.
The sexual assault charge and weapons charges were dismissed, though under state law Kean could be put on a criminal registry for kidnapping.
Authorities have described Kean as a deeply troubled youth who repeatedly fought with neighborhood children and was expelled from two schools.
The state Division of Youth and Family Services found that Kean's father, Kurt, physically abused him in 2001, but did not remove the boy from his home. Kurt Kean has denied the charge.
Amir's adoptive mother, Rosalyn Singleton, has sued the child welfare agency for neglecting Aaron Kean. Amir's aunt, Cynthia Smart, said Friday that the sentencing was a relief to the family and that she hoped Kean would "get some help."
"It's been very difficult. It really hit the family very hard. ... Something must have happened to him in order for a child to do something like this. Hopefully we can put it to rest."