Why don't you want any fanfic written about anything or anyone in your books?
The first and last reason, which is to say I don't need any more reasons beyond this one, is that my agent doesn't like fanfic. She says it's a very muddy copyright area at best and furthermore could in some cases be setting precedents that could cause trouble now or later as the whole enormous evolution of the net hurtles on in ways nobody predicted and nobody knows how to handle. Furthermore I could get in trouble if something that appeared in some fanfic somewhere seemed to show up in something I subsequently wrote. I don't read fanfic and when I've been sent it occasionally I toss/delete without looking — but I wouldn't be able to prove I hadn't seen it if it's out there.
I pay my agent to give me professional advice and she's good at her job. I'm not going to go against her about this. And the question — and its answer — really end there.
Having said that however, I will add that it's okay with me that there are good practical professional reasons to say 'no fanfic' because it makes me uneasy for a different reason. My personal feeling is that while using other people's worlds and characters as practise and inspiration is not only good but recommended — I did it myself, and you can learn a lot about the craft of writing by copying/plagiarising/borrowing/spinning off from books and writers you admire — and showing stuff you've written from these origins to your immediate circle of friends, family, teachers, creative writing group, whatever, as an exercise to improve your skills is also fine. And this would as far as I'm concerned (although if I get any queries about this I'd have to check with my agent) include any private, password-protected, invitation-only groups on the net. But using other people's work should only be an exercise in getting yourself going into your own work. (Or a private fantasy. What you do at your own computer, so long as you don't do it in the street and frighten the horses, is your own business.) And I feel that if you're going to display/hang/offer something for strangers, for anyone and everyone, to read — as on the fanfic sites — it should be your own work. Yes, sources, catalysts and retellings are always with us — but mostly it's pretty obvious where the line runs, and fanfic is by definition on the wrong side of the line.
I'm also old (born 1952) and my own gift for writing fantasy grows out of very literal-minded, pragmatic soil: the things I do when I'm not telling stories have always been pretty three-dimensional. I used to say that the only strong attraction reality ever had for me was horses and horseback riding, but I've also been cooking and going for long walks since I was a kid (yes, the two are related), and I'm getting even more three dimensionally biased as I get older — gardening, bell ringing (English change ringing on big tower bells), piano playing — and while you could argue that playing the piano is another of these arty farty creative things like writing stories, yes, it is when Andrea Hewitt or Mitsuko Uchida does it. Not when I do it. And my piano is highly three-dimensional — a great gorgeous 1897 Victorian thug of an upright. Nothing laptop or plug-in about her... and the wrong notes hurt. Ow. And the stories I seem to need to write seem to need that kind of nourishment from me — how you feed your story telling varies from writer to writer. My story-telling faculty needs real-world fresh air and experiences that create calluses (and sometimes bruises).
In September 2007 I started running a blog. It's kind of got away from me, for both good and ill, and I'm a heck of a lot more online than I used to be, and not only do I spend a lot of that time nervously re-checking stuff I'm about to post, I have enormous fun following the links that readers send me. But the underlying truth is still that I'm not a real Internet girl and my dubious new skills remain rudimentary. Some of my equally elderly friends are great whizzes at/on the Internet, blog away like mad, embedding videos and sound clips and little interactive gizmos as part of the entertainment. Not me. Posting photos is a stretch and while I can get to Google I will still most likely promptly get embroiled in the wrong search. I'm not built for the new etherware world, and I've been marching in the wrong direction for over fifty years. Virtual to me is only . . . virtual.
I remember when mimeo'd pages of early fanfic about Captain Kirk and Mister Spock were being passed around at cons - but even then I was already thundering off in a different direction. And passionately as I adored the original Star Trek (hey, I was in junior high when it first came out: it was brilliant, even with the rubber lizards and Shatner's girdle) I wasn't interested in somebody else's scuffles with it. The Internet explosion of fanfic is an alien concept to me. I don't see why anyone would want to spend any more time in what is essentially someone else's work than they absolutely have to, to get on with their own. It is, to me, a kind of virtual virtual, and the fade from the real becomes kind of extreme. I don't get it - and because of what my agent says about it, I don't need to get it.
So no fanfic. Sorry.