I don't know. I've been involved with online fandom in some shape or form since I was 13 - and I'm 28. I think it's just the type of people the kids are (sports-oriented or more likely to want to stay inside), how much tech access they have, and what type of home environment they come from. Not to mention whether they're already geek inclined - I mean, I talked my parents into taking me to a Star Trek convention before I was 10.

I think it's more that fandom is now a whole lot easier for people who aren't, well, the type of baby geek I was, to find. It's no longer something you have to specifically seek out. Which also means that a lot of younger fans aren't sticking around for decades like they used to. Those of us who are basically lifers are no longer the dominant force in newer fandoms.