More than one in ten boys and one in seven girls have had sex by the time they turn 15, according to a report released on Thursday.
The report was compiled by the Human Sciences Research Council from data collected in 2008 for a national HIV prevalence survey.
The survey covered 15,000 households across the country.
In the 12 to 14 age group, 10.8% of boys, and 14.5% of girls, said they had already had sex.
Most boys in the survey - which covered children from birth to 18 - tended to have sex with partners in their own age groups.
However more than a quarter of girls had most recently had sex with males who were five or more years older than themselves. This put the girls at risk of contracting HIV at an early age because older male partners were more likely to be HIV positive.
Among those who were sexually active in the 15 to 18 year group, just under one in three had had more than two sexual partners in the last year. The figure was lower - 9.5% - for girls.
However it appeared that most of this sex was protected by condoms, with usage at last sexual encounter as high as 92.1% for boys.
HIV in infants
The report also said HIV prevalence among infants from birth to two years was 2.1%, lower than the 3.3% average in the age group birth to four years.
This could indicate that the mother-to-child transmission prevention programme had had a positive impact in the two years before the survey.
In that period, coverage had increased significantly to reach almost three quarters of HIV-positive mothers with antiretroviral treatment.
The lowest HIV prevalence among all children, not just infants, was in the Western Cape, where it was less than 1%. KwaZulu-Natal stood at 3.4%, and Mpumalanga the highest of the nine provinces, with 4.5 percent. - (Sapa, May 2010)