The demographics pane displays stats based on sex, age and interests. How did it learn this private information?
Google can determine user's gender and age when an user provide his name to a google account or to a google partner. Having an user name is easy to discover the gender using a service like Dots Name Validation (http://www.serviceobjects.com/products/demographics-more/name-validation).
Also many visitors uses a google account where their provide informations like gender and date of birth.
As you may see, the information is not 100% accurate. It's just a small estimative.
They don't.. I manage an Arabic baby web site where 90% of the users are supposedly male.. hmm? So male Arabs are looking at a baby website???! NO they probably have wives who log on using their accounts.. It's a meaningless statisitic