* Fast-growing companies at large scale do enormous amounts of college recruiting. That's because the quality of the pool is much higher than the pool of out-of-work programmers... not because young people are smarter or better programmers but simply because the candidate pool includes a cross-section of skills, whereas the candidate pool of out-of-work programmers is biased to programmers who can't find work.
See also: http://www.joelonsoftware.com/articles/FindingGreatDeveloper...
Thus, the average age at Facebook is younger than Google, at Google it's younger than Microsoft, and Microsoft is younger than IBM. It's just a question of when the biggest growth spurt occurred in hiring.
* Nobody was working as a software developer 30 years ago. Well, of course, not nobody, but this field has grown like CRAZY. Everybody knows that, but they don't think of the implications. Even if every software developer who started work in 1980 was still working today as a software developer, they would only constitute a small fraction of the current workforce because the number of software developers in 1980 was miniscule. And you would expect them to be working in relatively mature companies simply because those were the companies that were around when they got their jobs.
Bottom line--this "mystery of the disappearing tech worker" is a good bit more imaginary than real and is probably mostly explained by the kinds of people you're hanging around with, not a real social phenomena.
See also: http://www.wolframalpha.com/input/?i=computer+software+appli... (If anyone can figure out how to get older data than Wolfram Alpha displays, I'd appreciate it)