Airlines euphemistically call this yield management. Advanced pattern-recognition algorithms compare the current booking status of any flight against previous data and predict the expected value of every open seat on the plane, in some cases on an hourly basis. The system then decides whether to make a discount fare available or hold out for the full sticker price. Differential pricing is a common practice in the world of atoms. Seniors get discounts on London buses.
Children get into Disneyland for less than adults.
Student passes make European travel affordable. Yet this almost never happens when using online services, in part because the industry is young and in part because authentication is difficult - though not impossible. Knowing who the user is will afford new and attractive solutions for selling and protecting intellectual property. For example, a child doing homework will be able to use this back page free, while an adult consulting it for a business plan will need to pay. Actually, it is.