It's nothing particular to the current generation of 18-25 year olds, it's just that young people have different, often less sophisticated interests. If I suggested that you go hang out with a bunch of kids from your local middle school, how would you feel about that? It's probably not high on the list of things you want to do, simply because the middle schoolers are immature and don't share your interests. The same is true of older groups with respect to high school and young college aged kids. It's nothing personal, and there's nothing wrong with being 18, it's just that we would prefer it if you left us out of it.
The key thing here, is that it's not bad to be a normal 18 year old. You (well, probably not you, since we are having this discussion) should enjoy it while it lasts. For you in particular, the anonymity of the internet is your savior. Just don't tell anyone you are 18, and no one will be the wiser.