Nkosi was born with HIV and was the longest surviving child born with the virus in the country.
Nkosi Johnson, who collapsed with Aids-related brain damage in December, had been praised by Nelson Mandela as an icon in the struggle against HIV/Aids.
Former South African president Nelson Mandela said he was an example for the whole world to follow.
"It's a great pity that this young man has departed. He was exemplary in showing how one should handle a disaster of this nature," Mr Mandela told reporters.
"He was very bold about it and he touched many hearts," he said.
Nkosi will be particularly remembered for a speech he made at the World Aids Conference in Durban last July.
"You can't get Aids by hugging, kissing, holding hands. We are normal human beings, we can walk, we can talk," he told delegates.
Catalyst
BBC correspondent in Johannesburg Allan Little says that Nkosi had shaken South Africa, where one in nine people are HIV positive, out of a state of deep denial.
Nkosi was born with HIV and was the longest surviving child born with the virus in the country.
His mother has since died of Aids, and he was adopted when he was two by a foster mother, Gail Johnson.
Speaking before his death Ms Johnson said: "We chatted about death... He had strong feelings about letting me down," she said.
"I told him I would miss him and no one could take his place."
In the absence of money for drugs, Nkosi had survived on a healthy diet, vitamin supplements and minimising the stress of being HIV positive.
Ms Johnson attempted to give him a purpose by turning him into a spokesman for Aids awareness at a time when the country's infected population was silenced by the suffocating stigma of fear and prejudice.
"I feel I am going to die quickly, like my mother died, very soon," he told the BBC. "But at least she got to be a grown up. I hate having this disease."
Ms Johnson has said: ''He's given Aids a face and allowed people who are still afraid of being associated with Aids to grieve openly. Most importantly perhaps, his fight and his bravery have given hope to many, many people."