When sexual abuse at 13 and older has been a part of someone's real life, stories like this become extremely distasteful.
If it is all fantasy, I suppose it is no different from the rape of a twenty year old. Rape fantasies are rape fantasies.
What age is too young? Well we can debate forever on that. Yes, as has been pointed out, it all depends on the country, the girl, the time in history.
But, in my opinion, what it all boils down to is this: if the writer portrays the female as a child and not as a grown woman (pay attention to the description of the female to make this decision), then its child abuse and child pornography, period, no matter what the age.
I read a story in the library that had a 14 year old, but the description of the female from her physical appearance to her mental disposition was that of a child, a little girl, innocent and unaware of sex. In my opinion, that story was a story of child abuse. I don't care if the girl had been 18. She was described as a child. The author put an arbritary age out to pass the criteria for publication. I read this story to the end to see how far this author would go. The last chapter described the rapists' plans to get a 9 or 10 year old next. I think this one may have slipped by Jinn.
There is something to think about.