One of the brothers accused of killing Damilola Taylor had previously admitted stabbing him, the Old Bailey has heard.
Damilola bled to death in a stairwell Peckham from a leg wound
A witness, who cannot be named, claimed Rickie Preddie, 19, told him: "Yeah man, I did it".
Rickie Preddie and his brother Danny, 18, of Peckham, south London, are being tried for manslaughter for a second time - a charge they both deny.
The witness said the conversation took place at a children's home after Damilola, aged 10, was killed in 2000.
"He was intoxicated, under the influence of cannabis, but he was in a fit state of mind to understand what he was saying," he said.
Damilola was found bleeding to death in the stairwell of a Peckham housing estate in November 2000.
The 21-year-old witness had given evidence in the previous trial of the Preddie brothers.
The pair were cleared of murder and assault with intent to rob, but the jury failed to reach a verdict on the manslaughter charge.
The case continues.