It's mostly a guess. My entire rlist is pretty much in their 30 and 40s and when I expand that out to my network, it also tends to be people in their 30s and 40s with the occasional venture up or down (which could certainly be all of us compounding selection bias). On top of that, my observation of the initial user base often came from a part of livejournal fandom 10 years ago, which was mostly late teens to 30s or earlyish 40s and the tumblrers I've seen migrate also seem to be more 30s with a smattering of 20s. It's very non-scientific.