I've worked in tech in Silicon Valley for 15 years, both as an engineer and as an engineering project manager. I've worked for a small startup, a small startup that became a medium sized company and then acquired, and two ginormous companies.
So, anecdotally....
Ethnicity: At my first couple of companies my engineering coworkers were usually white. At the ginormous companies there were quite a few Asians also, plus whole teams of Indian contractors (I only met a couple of contractors that weren't Indian). Of the literally thousands of people I worked with in all of 15 years, only two were black (both were in project management, also, not programming).
Sex: I've never had a female manager; I believe I've worked alongside three my entire career (and one was in project management, which tends to have more women in it than straight up engineering).
Age: The vast majority of people I've worked with have been between 25-45. Apple was a standout in my career for having both college grads and people close to retirement age (there are less-than-noble reasons for both, I fear, but at least those people did exist!).
I have a lot of conflicting thoughts about the problem in general. It's an issue I've thought a lot about, ever since college (I know I've mentioned that my graduating thesis in 1999 was about women in Computer Science). I think there's a lot of chicken-and-egg going on...there aren't a lot of women and minorities in tech in Silicon Valley because there aren't a lot of women and minorities in tech in Silicon Valley. People tend to hire other people like them. This was blatant in my project management team at Apple, where the hiring manager was a metrosexual (but hetero) white man in his mid 30s and, by the time I left the team, almost every single person he'd hired was a metrosexual white man in his mid 30s. We used to joke that they had bromances, they were all so similar. I don't think he did it intentionally (or at least, any exclusion wasn't done consciously) but it was incredibly blatant.