"(Norman Matloff is a professor of computer science at the University of California, Davis. The opinions expressed are his own.)"
Ya know, If I was a prof of CS at UCD, I'd probably think my upward mobility was limited too. I interview fresh college grads and senior professionals alike in my software company. I, personally, am equally likely to pick either. Age isn't as much of a metric for ability as ability is. 20somethings have lots of energy. 50somethings have lots of experience. A good team needs both.
Facebook is an experiment. It's unclear how successful they will be as a company. I do know people that work there and youth is a highly regarded trait.
MSFT is a failed experiment. A company like that is where talent goes to die, in my opinion.