Teenage girls experience high rates of sexual assault. The Criminal Code permits the Crown to substitute proof of young age for proof of non-consent for sexual assault and related offences applicable to young complainants. This paper focuses on the defence of mistaken belief in age. It provides a defence where the accused honestly believed that the complainant was at or above the age of consent and where the accused took all reasonable steps to ascertain her age. A review of the cases considering the defence indicates that it is often applied incorrectly, where the accused does not have any belief as to the complainant's age, or where he has not taken any steps to ascertain her age beyond a visual observation. These cases are vulnerable to stereotypical reasoning that girls are ''old enough" based on appearance, dress, alcohol consumption and prior sexual experience. Even in cases where the accused has little reliable information about the complainant, he may be entitled to proceed with the information available at the time, rather than being required to refrain from sexual activity. The paper argues for a reconsideration of how the defence is applied to bring it more in line with other developments in sexual assault law.