In that case I'd say that your age demographic is mainly just a side effect of internet literacy/usage rates and the randoms that are invoked. In general younger people are both more likely to use the internet and use it more often then those who are older. Combine that with the fact that most randoms are aimed at a particular group and it intensifies the effect further (since, for example, a star trek slash fix won't appeal near as much to non-trekies), meaning our slight teen weight becomes a much stronger one. You might also mention that a larger portion of "dabbling" writers are younger, instead of older.
As for a gender divide, I'd point out the known fact that (as a broad generality) women are the main consumers of written erotic material, while men are the main consumers of the visual stuff. Many more women reading the stuff means that even if both genders had the same percentage of slash fiction writers/readers you are going to end up with many more women slash fiction writers.
Put the two together and you are going to end up with a strong female teen basis, which is really just a side effect of a few other smaller things. As we can see with 50 Shades, take the fandom aspect out of it and put it in a format that is more easily accessible/understandable to older generations and they'll eat it up just like anybody else will.