No aog, don't think it would be smaller if it were an adult woman!It might be greater. The small disparity in age counted in this defendant's favour together with his own age of 13 at the time. Courts work, and to a large extent the laws bind them, on the basis that young offenders may not grow up to be adult offenders. But in exceptionally serious cases, the court may sentence much as they would an adult offender, the place of detention being different.
When you were young and in short trousers, the law did say that you could not be convicted, as a principal, of rape, because there was a presumption that a boy under 14 was incapable of the act. Can't recall, offhand, when the law caught up with biology, but I feel sure it was later than 1970.