Knowing that different users come with different preconceived notions and expectations is extremely important to the development and implementation of interactive design. It’s simple to assume things about one’s audience and create a site based off of those assumptions, but without hearing from the source, is it truly an accurate reflection of the audience’s expectations?
I think that, of any group, people in their late teens and early adulthood (~16-25 years old) are often the most confusing group to create for and cater to. They’re very likely already acquainted with the internet and digital media, which only contributes to the broad range in tastes and experiences of teens and young adults. It’s impossible to create a single site that will totally meet the expectations of such a diverse audience, but there are elements that most of that audience may identify with.
It might be interesting to look at design for millennials as a topic of exploration.