Last time, we talked about Twitter as a promotional tool, and ended with a joke about not using it to target David Letterman.  It turns out, though, that you might be better off targeting Letterman than a teenager, because new numbers indicate that Twitter isn’t popular among kids.

Note: the fresh report doesn’t challenge the idea that Twitter’s a good marketing tool.  So we’re not doing a 180.  This is just to help you refine your use of the social site.

Nielsen’s David Martin and Sue MacDonald stated, “In June 2009, only 16 percent of Twitter.com website users were under the age of 25.  Bear in mind persons under 25 make up nearly one quarter of the active US Internet universe, which means that Twitter.com effectively under-indexes on the youth market by 36 percent.”

Meanwhile, 64 percent of Twitter’s users fall into the 25 to 54 age group, and 20 percent are 55 or older.

So if you want to try to make money using Twitter, just carefully consider what sort of people it is you’re trying to reach.  Martin and MacDonald wrote, “The fact remains that Twitter has grown to be a major online presence and is being driven forward by significant buzz.”