In my observations over the last two years or so (didn't spend much time online before that) I've seen people all across the age spectrum. I have noticed however that the 18-30 crowd is often filled with less serious minded people when it comes to BDSM. Before the people that age come and try to beat me up, let me say that I have had an interest since the age of thirteen (active at seventeen) and I'm twenty-seven myself and still very interested. There are always exceptions, but it seems that the 30+ crowd know what they want and are willing to work for it. I have seen numerous 18-30 year olds disappear from forums, chatrooms, personal sites, and even one on one relationships. I think that is probably normal because these people are still trying to find out who and what they are. I have also seen a number of 40+ women who are just starting because they are finally allowing themselves to explore and, like lucy said, they have been stuck in relationships that are unfulfilling. It may also just be a matter of logistics. Who is more likely to be online? A twenty year old who has no kids, no real career responsibilities, and no desire to stay home on Friday night; or a thirty year old with a child, career and wife (husband) to be with?

Of course the above could be complete nonsense, but it's my two cents.