Goggle Analytics can create reports based on age and gender of a web sites visitors. Here is how they get this info.

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How Google determines demographic information

When someone visits a website that has partnered with the Google Display Network, Google stores a number in their browsers (using a "cookie") to remember their visits. This number uniquely identifies a web browser on a specific computer, not a specific person. Browsers may be associated with a demographic category, such as gender or age range, based on the sites that were visited.

In addition, some sites might provide us with demographic information that people share on certain websites, such as social networking sites. We may also use demographics derived from Google profiles.