I have seen a couple of threads on a similar topic to this at another site over the past years.

One dealt with a story about the Olsen twins, the day after their eighteen birthday. (birthdays?) At the time they would have been about thirteen or fourteen. The other was regarding the dozens of erotic Harry Potter stories that many people, at the time of the first movie’s release, seemed obsessed with. Again the authors projected the cute kids into the future to avoid the issue of underage sex.

Personally, I don’t have a problem with fan/fic unless it involves celebrities who are underage at the time the story is written.