I don't know a lot of this sounds like reverse ageism. Facebook was built entirely on young, talented engineers who could perhaps scale out software better than "experienced" developers. Same goes for a lot of Google products. The barrier of entry for software development is much lower and you can pretty much hit the ground running with a few good math and algorithms courses and good internships. You can become a good software architect with a few years of experience -- even the best are barely 50 years of age (Jeff Dean, for example).
I am not sure why this trend didn't translate towards hardware but if I were to guess - the jobs are fewer, the barriers of entry are higher (no one really teaches hardware design in school) and the cost of mistakes is bigger.
No rational person that has lived through age discrimination would engage in any kind of ageism and again, my career spans over 40 years. (How many people my age in the Fortune 500 company 40 years ago? ZERO! Maybe down in mailroom or something, but they weren't even allowed in the tech areas so I would have to assume.) I also strongly disagree with your assertion that Google has "young" engineers developing "a lot of Google products", at least in the sense that they are doing so without guidance from experienced engineers. As far as Facebook, I don't find their stuff very interesting (though I thank them for helping to all but destroy HP with their open hardware) --it's a website with a big power bill and a non-ACID database, right?
I am not sure why this trend didn't translate towards hardware but if I were to guess - the jobs are fewer, the barriers of entry are higher (no one really teaches hardware design in school) and the cost of mistakes is bigger.