Here are some rules of thumb that may help guide your approach.
In my experience, you need to divide up your problem into two different issues to solve: design and branding. Design for functionality and smoothness of user experience. Brand so that your audience relates well to your product.
A lot's been written about good design. Yes, teens definitely respond to it. If a site's hard to use, they have the attention span of hummingbirds and if you cause an engagement failure because of poor usability, they'll be gone and won't return. Above all else, focus on making your site easy to use and the user experience seamless.
Branding depends on your audience's demographic. Most teens have developed a strong enough, though flexible, identity to be attracted to some things and repelled by others based on their perceived sense of who they are in their greater and lesser social ecosystems. You might be interested in reading everything danah boyd (intentionally lowercase) ever wrote, most especially "Viewing American class divisions through Facebook and MySpace" here:
http://www.danah.org/papers/essays/ClassDivisions.html
In general, people, no matter what their age group, adapt to symbolic systems they're immersed in. Kids in gradeschool can glance at a page in a textbook and from the fonts, point size, layout, graphics, tell you precisely whether it's a third grade book or a fourth grade book, and therefore whether it's for them or not. (The same goes for someone looking at a full page ad in The New Yorker - font, point size, color choices, etc..) Higher income demographic kids are exposed to high end design by the time they're in their teens and have a more positive response to it than kids without the opportunity and exposure to design. In my own work with teens, non-college bound teens find Facebook's design BORRRRRING! [picture an entire class of kids singsonging that in unison] They tolerate a _lot_ of busy-ness and chaos on a page and will jump onto MySpace whenever your back is turned.