In a blog called Influential Marketing, Rohit Bhargava makes a great observation about demographics: They suck. Actually, that's not exactly what he says. What he does say is that looking at age as a demographic is not very useful. You can read his post for the details, but he echoes a point I've made over and over to anyone who will listen (that means you, dear reader) - demographics are only the beginning of understanding your customer, not the end.
Too often, clients tell me what they want us to help them do. Often something exciting like "sell more!" or "create a viral video!" Then I start asking questions about strategy and target audience and spoil all the fun. If the answer is along the lines of "males 18-34," then you're talking about a huge, diverse, group of people with a wide variety of tastes, priorities, values, and interests. The list of brands or products that will appeal across the board to a group like this — or any significant demographic segment — is very short. Chances are you're name isn't on it.
What that response really tells me, and apparently Rohit, is that you haven't finished your homework. You should be able to answer questions like: What kind of music do they like? Are they conservative or liberal on social issues? Who are their favorite clothing designers? What do they do for fun? Do they prefer PlayStation or XBox, or even Wii? Do they like shooters or sports games?
You may think these answers aren't relevant to you selling your product, but let's remember, if you can't engage consumer in conversation, you won't get the opportunity to market to them. How will you get someone into a conversation when you don't know anything about them?