If you’re looking for an audience predominantly under 25 years of age to market to, Twitter may seem like today’s obvious choice.  Only one problem: it isn’t.

For all the hype the microblogger gets, only 16 percent of U.S. visitors to Twitter were under the age of 25.

Business Insider posts an interesting chart (shown right) showing how, in general, Internet sites average 25% in that age category, with Twitter showing weakness in that valuable demographic by 36 percent.