Microsoft Adcenter Lab’s Demographic Prediction tool is pretty interesting. I have no idea how accurate it is, but it’s informative to compare one site or one term to another. This tool predicts a user’s age, gender, and other demographic information, based on their online behavior, such as what queries they search online and what web sites they visit.
I was amazed to find 25% of visitors to our site are between 18 and 24, until I remember all the job opening requests we get from recent college graduates.