The sexual exploitation of children and adolescents is a frequently underestimated health problem which includes transactional sex (TS), or the practice of sexual activity based on an expected return of benefits, favours and/or support in some form. This qualitative study focuses on age-disparate transactional sex (ATS) in urban favela communities of Rio de Janeiro between adult men (over 18) and girls and adolescents (G/A) (under 18), involving a minimum 5-year age disparity. We have employed social norms theory as a framework to identify the prevailing social norms contributing to or protecting children and adolescents from these relationships. Data collection utilised semi-structured interviews (n = 30) and ten focus groups with a total of 130 men/boys and women/girls selected through purposive sampling and varying in age from 15 to 65. Overall the findings identify factors, especially the essentialisation of gender, which promote the acceptability of ATS. When ATS surpassed the acceptability threshold, social norms discouraged direct interference. Concluding remarks point to possible strategies for reducing the occurrence of ATS. These must include girls, boys, women and men with community involvement in the deconstruction of social norms involving gender, age and economic consumption.