An examination of age and sex or gender (sex/gender) in HIV vaccine preparedness studies can contribute to an understanding of these demographic variables in preparation for actual HIV vaccine trials. In this descriptive review, age and sex or gender (sex/gender) were examined in relation to willingness to participate (WTP) and retention in an HIV vaccine trial. Twenty-five articles were retrieved from the Organization for Economic Co-operation and Development (OECD) countries and 28 articles were retrieved from the non-OECD countries. In US studies that involved mainly white MSM, older men were more likely to be WTP in a hypothetical HIV vaccine trial and more likely to be retained than younger men. In most OECD studies, sex/gender was not associated with WTP in a hypothetical HIV vaccine trial, while females were more likely to be retained in most studies. Largely, age was not associated with WTP in the non-OECD countries, but the results on sex/gender were more variable. The relationship between adolescent or adult WTP in hypothetical HIV vaccine trials in South Africa did not appear to be modified by high school student status. In addition, more studies in discordant couples in the context of HIV vaccine preparedness could be conducted to examine gender roles and inequalities in preparation for HIV vaccine trials.