Ontlametse was one of the two South African young women living with Progeria, an extremely rare genetic disorder of rapid ageing at a very early age.
She died last Tuesday at Dr George Mukhari Academic Hospital in Rankuwa after experiencing breathing difficulties.
Doctors did not think Ontlemetse would live to see her 14th birthday, but on March 25 she turned 18 after finishing her matric. Progeria sufferers rarely live beyond their teens, and very few survive into their 20s.
Last week Wednesday, during his birthday celebration, President Jacob Zuma presented Ontlametse's family with a car. Zuma spent a day last month with Ontlametse to celebrate her 18th birthday.
Ontlametse's wish was to see her family living in a decent house and having a car. The family had been invited as guests to celebrate Zuma's birthday.
The funeral will take place on Friday in Hebron.