I had an interview at Google a couple months ago and noticed that most people were pretty young. When I asked the person who was in charge of taking me to lunch about this, he said that it's probably because there are just much more graduates of CS now than there were before, and that Google would very much like to hire senior people as well but they're much harder to find.
I wonder how much of what he said is true vs ageism.
On the other hand, I wonder how much of a natural bias there is against older people if they have to go through the same interview process because it felt like a mental marathon to me. Although the interview only lasts a day it took a couple days for me to recover.