Advertising sophistication

I was trying to set up business with a medium size CPM advertising network and in a phone conversation, he asked me: "Tell me about the demographics of your users."

I candidly and stupidly replied. I don't know much about them, I think they're mostly kids.

BONG. Wrong answer.

You should pretend to know something about your users to have some credibility. It's not really checkable information so there's really no wrong answer. And, grown-up users are more valuable than kids. So I got zero out of two on that little test.

But how would I know about my user demographics? One, if your site is registered with a service, just repeat their information. It might be nonsence but it sure sounds good. So, for instance, for my SpellingCity.com site, I could say:

My site is 62% female, 49% over the age of 18, and 50% of the users are regulars. I get all this information directly from the Quantcast website. I'm not sure I believe it but this sort of info is the standard medium of advertising.

I'm also trying to understand how comscore with media metrix provides info so that I can better understand and characterize my site, and find appropriate advertisers. I was just reading through one of comScores PDFs which explains how to characterize your site and it baffled me.