Either everyone answering is really young...or some people really stick with one job for years?!?!
I had five 'proper' jobs. None for more than five years. Around the age of forty I was made self-employed. And that's where I still am.
I don't think it's that everyone on Slashdot is young. It's just normal employment doesn't last very long any more, unless you're one of the handful that gets promoted to senior management. Once you're over a certain age, hopping from company to company becomes a lot less easy - what's thirty years' experience against a bright young poppet with a shiny new MBA?
This is partly sour grapes, but tax rules, pension deficits, redundancy liabilities and the cost of health insurance would all count against hiring older employees, even if simple ageism were extinct.
There are exceptions, of course. But, if you are still in your twenties or thirties, flitting from job to job, it's probably worth thinking about how you'd earn a living for yourself when the time comes.