I agree there is ageism in Silicon Valley, but it's present in many other industries as well. As for the low number of blacks and hispanics, that shows a problem with our education system. Perhaps with our society as well but it's unfair to blame it on tech companies. And startups with sh**tty cultures do exist but that doesn't mean all places are the same.
The problem with our education system is that blacks and hispanics (not all of them, but a disproportionate number, and we are speaking in statistics not in absolutes) refuse to make the slightest effort to apply themselves in school and are instead often not just disruptive but outright hostile. The schools aren't failing these kids. These kids are failing the schools. Every teacher in America goes out of her way to help and accommodate these kids anyway because that's part of the job, but you can't fault business, especially ultra competitive information driven business for not doing the same. It's not part of their job.
This is excessively categorical, even I have to admit.