I had a short discussion about this with a friend today, who is professionally involved in substance abuse among young people, and she pointed out that the brain of an adolescent is still developing at that time, so they don't recognize the consequences of their actions ahead of time. In fact, the brain continues to develop until the age of 25, and I know a few folks who are way past that point, who still act as if they were 15!
We probably need to go case by case, however if both parties were consenting at the time, then it should be considered at that level.
Assuming that the 13 year old is not already flirting with or deep into drugs, drink, and other dangers, which would not be a good scenario for any sort of consent when it comes to sex, 13 does seem to be the age at which a sexual identity begins to emerge for girls at least, and they begin to create a persona that is no longer a little girl, but a woman in the making. And this is the crux for society to determine what is right as a matter of public policy, or to create a senseless witch-hunt.
Jerry Lee Lewis was unable to make concert appearances in the UK in 1958, because he was legally married to his a) first cousin (once removed), Myra Lee Brown, and b) Myra Lee was 13 at the time. Jerry Lee was 28. They remained married for 13 years, despite the scandal that ensued. It wasn't easy street for Myra, but she was considered of an age to make that decision. Women in their 20s and upward have made the same decisions as this 13 year old woman.