According to a study published in Clinical Infectious Diseases, approximately 20 percent of children who have been living with HIV since birth had not yet learned their HIV status by the first time they had sex. Even more worrisome, a substantial fraction of those teens who did know their status didn't disclose their infections to their partners or choose to consistently practice safe sex. This suggests that either they weren't aware of the risks to themselves and those partners, didn't care about those risks, or didn't have the skills needed to feel safe and comfortable disclosing their infections and negotiating safe sex.
Personally, I suspect it's mostly the third, perhaps with some of the first and second thrown in. HIV is a highly stigmatized condition that can be difficult for an adult to talk about with a potential sexual partner, let alone a teenager. Similarly, it's difficult for most teens to negotiate safe sex, even in situations less emotionally fraught than when they know they have HIV and are afraid of being rejected, judged, or even injured for an infection that was utterly beyond their control.
Mind you, that isn't to say that a lack of knowledge of HIV risk behaviors doesn't also play a role in the behaviors of some teens. Given that a significant fraction of the parents of HIV positive teens weren't comfortable telling their kids about their infections, it's hard to imagine that they would do much better when discussing the nitty gritty details of sex.