Mountain View? San Jose? Palo Alto? Which place has the best nightlife, etc. for someone under 25? San Jose is larger, is more going on there?
San Francisco. 
If you are working in Silicon Valley, you have a car to take you to the night life.
It’s been a long time since I lived there. Palo Alto has some night life, but it’s so expensive to live there you might as well forget it. Mountain View, Sunnyvale, San Jose are probably better bets. Maybe Menlo Park, too.
But in recent years, the kids I have known working in “Silicon Valley” have been living in San Francisco and either working there or taking the company bus down to headquarters. San Francisco has great nightlife.
Santana Row in Santa Clara is where the nightlife is.
Mr. B “lives” in Silicon Valley 15-20% of any given year. As JHS said, a car is a must have item in Silicon Valley. Parking is mostly free, unlike in San Francisco. Mountain View has a busy downtown with lots of restaurants. So does Campbell (there is an awesome Austrian restaurant there). Palo Alto seems to be mostly residential, expensive, and quiet.
I hope folks with kids in SV will give you more feedback.
Cost of housing is likely to be a significant consideration. So is convenience of commuting to work, due to heavy traffic jams unless one’s residence and workplace make use of public transportation convenient.
Consider Redwood City.
It used to be a slum, but it’s rapidly going upscale. Nevertheless, if you’re lucky, you can still find relatively cheap housing there (relatively cheap by Silicon Valley standards – which is obscenely expensive by the standards of most of the country).
If you’re spending your entire income on rent, you can’t afford to have a night life.
I second San Francisco. When I was 26 I got a job in San Jose, and searched exhaustively for any place nearby that would have any urban or young feel. I eventually moved to a shared house in Mountain View, and I was miserable. A month later I moved back to San Francisco and commuted and I was much happier, despite the commute. True, this was a while ago, but the feel of these places hasn’t changed that much.
It would not be as long a commute if the job is in Palo Alto or Menlo Park, and, of course, some jobs have commuter buses.
If you’re young and working in Silicon Valley, you live in SF and take the luxurious company bus to work, and you don’t have a car. At least, that’s the profile of many, many young tech workers.
When I was a young tech worker in Silicon Valley, I did not live in SF. I lived on the Peninsula. I never had a car. I got around by bike. We have perfect weather here. No need for a car, then or now. Nowadays people can use Zipcar when they want to go out of the area.
My kids live in San Francisco and have no car.
They really are living interesting lives in a very unusual culture, at a very particular point in history. It’s fascinating to me. I didn’t see all this coming.
Back in the Stone Age, DH lived in Cupertino. Never had a car…always used a bike to get where he needed to go, including work.
Yes, if you live in SF, then you do not need a car. A nice company bus will take you to your Silicon Valley office. If you live in Silicon Valley, you have to have a car to be able to socialize and get places.
@alh, out here, we think you are living in an unusual culture. 
^my bad, dstark
@alh,
Yes. You are bad.
I am pretty sure in another couple of trimesters, SF will become the absolute center of the known universe.
@alh, If your kids settle here, would you consider moving to California?
I encouraged my son to live in MV. When he was looking for housing, there were several articles aboutpeople standing at the bus stops to Google, FB, etc., protesting how the techies were pushing them out of SF. It’s a few years later, so maybe the protests have subsided.
On weekends, my son visited friends in SF and PA
dstark: That’s what the kids want to know. At what point in the original gold rush did all the east coasters decide their kids weren’t ever coming back “home”? At least a covered wagon won’t be necessary this go round. Even though I will point out it takes longer for me to fly to SF than to London or Paris. Which are in other countries.
It already is. After all, it’s where Starfleet Academy is located. 
@alh,
Do your kids want to live here or they are just here for the jobs?
I am trying to think. Where in California can you move to, be close to your kids and be surrounded by wild animals?
I think that is SF.