I'm curious! Oh, and don't use appearance as a reason here - let's see if there are other things and deeper reasons for things, because it's fun to think about sometimes.
It's only something I've noticed recently, but I think perhaps that people only see me as 'childish' because while explaining things, I revert back to using the most simple words, despite having an extensive vocabulary thanks to Literature classes in high school. It's not trying to be childish, but more trying to add all the feels associated with what you're trying to say, into speech. Since simple words can mean a wider range of things (as opposed to more complex words that are associated with specific things) I tend to subconsciously go back to that pattern of speech. Of course there are times I do this specifically because the people I'm talking to seem like their education never made it past a certain point, LOL.
Other than that, they might see me as childish because of my aversion to many things people around my age like (drinking, sex, partying, whaaat) or my restlessness/awe in places that seem to have so many things to do. What about everyone else?