Public perception of regulated sexual behaviour is a crucial factor in considering policy, particularly when UK sexual consent laws could criminalise the many young people who are increasingly engaging in underage sex. In contrast, consent age-spans have been progressively researched and implemented in other countries to respect sexual autonomy while simultaneously protecting young people from harmful sex. This study investigates UK perceptions of consensual underage sex between similar-aged adolescents, with predominant focus on age-spans, as an adaptive alternative to dropping consent age. An online experimental survey of members of the UK public (n = 336) manipulated age-gaps, perpetrator gender, and respondent gender. Supporting the hypothesis, results showed more support (i.e., reduced condemnation of the situation, perpetrator, and victim) was given for sex with age gaps of up to 2 years than those larger. Gender differences were present only for age gaps of 3-and 4-years; where harsher situation and perpetrator judgements were elicited when perpetrator was male. To our knowledge, this is the first UK-based study investigating perceptions of consensual underage sex, therefore, acts as a baseline and offers suggestions for future research. Results are discussed in terms of acceptability of behaviour and present