Cynthetiq nailed it. I've always been taken seriously by adults because I act mature (well, when I need to
) and serious. I'm 21 (look about 18) and have no problem talking to middle aged people about business stuff and, sometimes after some initial skepticism, being taken seriously. A big part of it is that I've been working since I was 13, so I have a lot of experience interacting with adults and knowing how they behave as opposed to my peers.