I think there are multiple reasons younger people (like myself) don't get very involved in posting.
1) This site CAN be intimidating to people who first come here (it was for me). The fear of posting something stupid was (and still kinda is) a big fear of mine, and I imagine it's similar for other newcomers.
2) Quite a few of us "young people" are in the chat room with a bit more frequency. Chat rooms are a more familiar form of communication to most people around my age, it's more instant and more direct (things many of us young people have always known) and forum posting can just be kinda slow.
3) Many of us young people have less experience in the world of tickling, and so we have less to talk about as we don't have much (or in cases like mine, any) experience to pull from and discuss.
4) The age group of mid 20's and younger consists of a lot of people in college. Which, at least for me, means that I just don't have that much time to be on the forum, at least not as much time as I would like. While I'm not trying to say "older" people have a plethora of time by any means, but college is an incredibly busy time, which makes for less time being on the forum.
5) This is similar to my first point, but it can be a little hard for some to get integrated into the community. This applies more for new members than specifically younger people, but as people first start to explore their interests at a young age they find places like this, so many new people are young people. But it can be hard to get integrated. If you're not a naturally extroverted person, making friends on a forum can be a bit difficult, and it can be hard to get things started.
These are just the first few things that popped into my head, and of course I can't speak for all people in my age group, but I think at least some of these apply to many people in my age group.