Ikeem Moore, 15, was found guilty at the conclusion of his three-day trial in Richmond County Superior Court. A sentencing date hasn't been set.
Although Mr. Moore had just turned 14 when the crime occurred, he faces the same punishment as an adult, a prison sentence that could be as long as the rest of his life. The minimum sentence is 25 years in prison without parole.
Mr. Moore was arrested in December 2005, when the victim first told her sister what had been happening after school, according to court testimony this week.
The child told her mother and grandmother, too, and later gave a child specialist a fuller account of the crimes.
Assistant District Attorney Rex Myers argued in closing Thursday that the little girl could not have given such a degree of detail about sexual activity unless she had experienced it.
Mr. Myers scoffed at the teenager's insistence during his testimony that he had never been alone with the girl or touched her in any manner.
Mr. Moore chose to experiment sexually with a 5-year-old girl, the prosecutor said.
Jean Colohan of the public defender's office argued to the jury that the account of molestation was planted in the girl's mind by a big sister who jumped to conclusions.
The preschooler described sexual penetration, but a physical examination proved the child had no physical injury or indication of abuse, Ms. Colohan said.
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