You should have as much info about your patrons as you can get if you intend to really market to them properly, and demographic info is incredibly important. In reality, you probably don't have time to do your own studies... but why should you? Lots of organizations gather and report on statistics. You might be thinking about Pew, the U.S. government Census, or... Newsweek?
Yes, newspapers and magazines often report on bits of demographics too. I found the Jan 26 edition especially interesting in this respect. The cover theme of "Obama's America: Who We Are Now" doesn't really explain that the issue is about the changing population of America. It's full of charts and graphs
that are really useful, and short articles about all sorts of populations.
My fave article was called "Campfire Questions" (p 66-67) and talked about how the Boy Scouts of America (BSA) was trying to recruit Latinos to join b/c the BSA's membership was dwindling and Latinos are the fastest-growing ethnic group in the country. Makes sense to target them, right? And if you just go speak to some community members, you could easily show how worthy the Scouts are, right? Not so fast...
Seems the Boy Scout recruiters didn't initially understand much about their Latino neighbors, and they made rookie mistakes in their pitches. One that you'll be able to relate to is the BSA simply translated a flier into Spanish and left it at that. Think it worked? (Does it work for you?) It didn't b/c the content of the flier, originally created for English speakers who already understood the Scouting concept, never actually said what scouts are or what they do. It highlighted ideals and things that scoutmasters thought were important, but it never defined BSA for a population that had no prior contact.
I recommend that you read this full article -- and study the interesting demographic info in the entire issue -- just to get a big picture of how demographics are changing and why it matters. Then look to your local government to get more specific info about your own area before you start doing any futher strategic or marketing planning. It's worth your effort to get your messages right the first time around!