I'm no expert at raising kids of course - I'm barely qualified to talk about the subject at all, as I don't have any nieces or nephews, either - but I was reading manga AND checking out porn by 13. I think a lot of people tend to forget how early you find out about that kind of stuff once they've grown up. My friends were teaching me how to curse like a sailor and find topless Playboy pictures online in 4th grade I think it was, lol. But that's only because I was one of those kids who wasn't allowed to watch R rated movies or anything like that from the get-go, while my friends could.
Without knowing any real details about the situation (like whether the kid's internet access is monitored, etc), the argument I usually see in these situations is:
"would you rather they get their access somewhere else?"
In other words, where there's a will there's a way, and one should assume it's gonna happen whether you've allowed it or not. Personally I think this site is one of the better options. It's free, comparatively safe (not only virus-wise, but there aren't download links all over the place either), no ads. You could do a lot worse, all things considered.