Migrants to IJ, 2007

It could be that a higher percentage in the younger brackets are at LJ for specific social networks. I know I'm still active on LJ in part because my friends from college are still active on LJ.

Older users may trend more towards fandom beyond general squeeing. If adults want to have adult discussions about characters, they will go someplace that will let them have those discussions. While right now the crack down is on visuals, what happens if the next wave is fanfic text and the wave after that is discussion text? Or if a sponsor doesn't like particular political of religious discussions?

The younger users also may not be used to the concept of having a real choice. Dislike your school? You're stuck. Hate your homeroom teacher? Too bad. Journaling service online breaking their TOS? What do you mean, leave?

There's a visible bump between 17 and 18 on the InsaneJournal stats, which to me would correspond to "you mean I get to CHOOSE a college, and I can transfer out if I have to?" or "first job after high school graduation" age. The discontinuity between 22 and 23 could correspond to college graduation; the cafeteria social network that could spread news about posts on lj_biz to those never hearing of that community but on LJ goes away then. And those who have been involved in fandom for years could account for the relative size of the upper age range.

(I would like to add, however, that larger sample sizes do tend to give smoother curves; IJ's bumpiness could be due to not having LJ's numbers.)