[bonoboTP]

The often cited reason is that young people are easier to manipulate and exploit (overtime, passion, kool-aid about the company "mission" etc.), or more charitably phrased are more "adaptable" and less set in their ways. They have fewer responsibilities, no family etc. They are more idealistic and don't have the same cynical/sour/realist/seeing-through-the-bullshit outlook that older people have.

[wtracy]

They're also easier to negotiate down on salary.