Take a look at Amazon's US patent 6865546. This is a patent for a system that determines a buyer's age according to previous buying records. The patent states that:
One embodiment of the present invention is a system and method of determining the age of an item recipient, such as a gift recipient. The age range for the gift recipient is estimated based at least in part on a customer order history of gifts purchased by the customer for the gift recipient. At a first date, a customer order for a first gift for the recipient is received, where the first gift is associated with a first age appropriateness designation. At a second date, a customer order for a second gift for the recipient is received, where the second gift is associated with a second age appropriateness designation. An age range associated with the recipient is then estimated based upon at least the first age appropriateness designation and the second age appropriateness designation.I would like to know the "age appropriateness designation" for my recent purchases. Let's see, I have purchased Cowboy Bebop, Hack//Sign, Neon Genesis Evangelion and Star Wars Knights of the Old Republic II. I bet the system would say that I am a 16 year old geek.