This could just be me, but I get the impression that people are getting access to fandom younger and younger. I mean, I'm only 21, but I'm still surprised when I see someone online who's under 16 or so, as I'd never even heard of the concept before then (and wasn't really able to get involved until I got my own computer at 18).

I think the rise in social networking sites are also a big factor-- they've created a kind of "tipping point"(Gladwell, anyone?) for younger kids to get an internet presence, and from there to get into fandom. Even though it seems like facebook has been around forever, it's really only been a few years, and it only became available for people not in college in, I believe, 2006. (I know there were plenty of networking sites before then, but I don't think they were quite so mainstream. I could be wrong, though.)