The demographics pane displays stats based on sex, age and interests. How did it learn this private information?
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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.