Last year I was 48. As part of something like a mid-life crisis, I interviewed at several of the Bay Area majors. In some ways, it was kind of a Logan's Run sort of experience, with me in the role of Old Man (Peter Ustinov). (Maybe next time I should bring some cats with me to the interview.) I was turned down by several, but received a good offer from Facebook. After a lot of careful number-crunching and soul-searching, though, I felt that I couldn't accept it. The primary reason is that I have a wife and kids. Though the offer would have been fabulous for a single guy, it probably would have been ruinous with my financial responsibilities.
I guess what I'm saying here is when discussing ageism and the Valley, one needs to be careful to pick apart reluctance to hire older people (which I don't doubt is a bias sometimes) versus the personal economics of the Valley, which makes it a marginal place to consider living for many people (and probably tends to hit families the hardest).
As an aside, I think many younger managers are nervous about hiring older workers. For what it's worth, I recently worked for several years for a guy that's at least ten years younger. Best boss I ever had. We got along and got things done.