They really like SF.
Gourmetmom: I think SF is a good choice, if the young person can afford housing and has a comfortable way to get to work.
We live in the Silicon Valley. DH works for one of the very large, well-known companies. Yes, tons of young people live in SF and commute on the fancy buses. But note that you can only do that if you work for Google, Facebook, Apple or a couple of the other large companies. If you’re one of the vast majority that work for a smaller company, you’re on your own to get to work. And be aware that SF’s rent are even higher than anywhere on the peninsula.
I’ve lived in Silicon Valley for over 35 years. I don’t have a car, yet I manage to get places and socialize.
OP, I’m assuming you are asking for your son or daughter? Another vote for San Francisco if they can find an affordable place to live – even if they have to commute to Silicon Valley. There’s really no other urban environment with an active nightlife for the under 25 crowd. If SF isn’t an option, Burlingame, Palo Alto and Mountain View have small downtowns that are thriving, but they are basically just a couple of streets within a few blocks. They might be “over it” pretty quickly if they’re looking for variety, but all have train stations right on the edge of downtown with trains that will take you into downtown San Francisco. In Burlingame, they’d also be a couple of miles from the Millbrae BART subway station.
But none of these places are inexpensive to live. Some are close to SF prices and I think it’s worth considering whether there would be much opportunity to meet other young people to do things with in these small cities. The best place to meet other young people with an endless variety of things to do would be in SF itself, and probably more options for shared living situations with other young people to cut costs - if that sounds like an option.
There is nothing worth noting going on in San Jose/Santa Clara for young people and that would be my last pick. Yes, there’s Santana Row, but it’s basically a shopping center with some very expensive apartments and condos above the shops/restaurants and not a place I’d describe as a place for an under 25 year old looking for an active social scene.
If they need to live that far south, I’d go 5 more miles south to Los Gatos which also has a downtown similar to the towns described above, but more active and you almost feel like you’re in mountain town because it sits right on the edge of the Santa Cruz mountains, and you’re only 20 miles from Santa Cruz beaches. But again, not inexpensive, same drawbacks as the towns above with the added drawback of being further from SF and having no nearby train station to San Francisco.
^^Tell us how far can you get places and socialize without a car. Getting members of your immediate family and friends to drive you don’t count.
CF bikes all over the country. 
Not everyone can do that.
@cbreeze A couple of people have said they get by without a car. I grew up in the area, still visit family there regularly (was just there last weekend), and in my view, the only place I wouldn’t want a car is if I lived in SF. It’s similar to NYC in that respect and sometimes having a car there is more of a pain than not. In any of the other place outside the city heading down the peninsula toward Silicon Valley, there’s no easy way to get around - unless you want to Uber everywhere. If you can get to work by foot, Uber, bus or bike and were close to a train station to get to the city, it’s doable, but I think I’d still feel somewhat trapped, especially if I were under 25 and still wanted to go out and do things all the time.
My son has lived in Silicon Valley with no car for about 4 years. He bikes to work. He takes the train to SF. He uses Uber, ZipCar, or rental cars whenever his bike and the train don’t meet his transportation needs. He has a social life, and he seems to be able to get where he needs to go. He could afford to have a car if he needed one. He says he doesn’t need one.
I stand by my words, Marian. Not everyone CAN bike to work for many reasons (lives too far from work, needs to wear business clothes to work, no shower facilities at work, etc.). At Mr.'s company located in the heart of Silicon Valley, 95% of people drive to work, and that includes the younger generation.
I think Mountain View would be a good spot for a new college grad to live.
@BunsenBurner, I agree with you that practically everyone in Silicon Valley would need a car. My son is a bit of a freak. He also lived in Los Angeles without a car.
Lol, Marian. SV is a good place to bike to work. Not as much rain as in my neck of the woods. Uber and Zipcars etc. definitely help to make car ownership much less of a necessity, but they do not completely eliminate the need.
S took our old minivan out to SV. I don’t think he uses it much, because he bikes to work. But I think they have gone to Tahoe several times and he drives there. He lives in MV with three other guys from the midwest, one of whom he knew at U of Illinois. They rent a three bedroom condo with an office, and he has converted the office into his bedroom. My son is a “roommate” type of person, so it works well for him, I think.
My kid lives in Mountain View and doesn’t drive. There’s not a whole lot of night life that I can see, though some cute shops on Castro Street. He likes his location because it’s near a Google bus stop. 
S1 lives near Castro St. in MV. Takes the company bus to work (about 2.5 miles), often walks home in the afternoons. Does not have a car, but goes to SF and PA regularly. He used to live in Sunnyvale, about 3/4 mi from the Caltrain and shuttle stop. Rent was a little cheaper there, He takes public transit or Uber/Lyft, depending on the location and time of day. He doesn’t do the partying thing, but does have other interests. He has a room and bath in a 3 BR apartment, so his share of the rent isn’t too horrendous (it is nonetheless more than our mortgage). He still lives like a college student and has managed to squirrel away a nice bit as a result.
Most people appear to be inclined to avoid exercise, even if avoiding exercise takes more time or is less convenient (e.g. waiting for an elevator that is slower than using the stairs). So even those who have no impediments like having to wear dressy business clothes (Silicon Valley companies are less likely to be dressy workplaces) do not consider commuting methods that involve walking or bicycling more than a trivial distance. So, while the more athletically inclined can have a significantly wider range of non-car commute options, such people are probably seen as outliers.
http://fortune.com/2015/10/23/is-silicon-valley-bad-for-your-health/ indicates that Silicon Valley is not all that unusual compared to the rest of the US in unhealthy habits. The question the OP should be asking is if the son in question is typical (exercise avoider, likely on the road to health problems as well as being more car dependent) or an outlier in this dimension.
From reading this thread, I am getting the idea we may have more CC family members living in MV than any other place in the country except for NYC.
My kid lives in MV and…
My kid lives in MV and…
My kid lives in MV and…
I think some of you are lying.
The population of MV isn’t that large.
MV is Mill Valley, right?
I guess I probably saw everyone’s kid in Mountain View on a Friday night eating at one of the restaurants on Main Street (or whatever that street is called).
Ucb, not every employer in SV is a tech co that loves to see their employees in stinky bike clothes. There are plenty of law firms there, just saying… And at Mr.'s co, managers are required to wear khaki/polo shirt kind of clothing, so biking is inconvenient for that particular reason. It is not because everyone wants to avoid exercise on purpose… 8-|
which is why many offices have showers.
Normally not more than 40 or 50 miles. Well, occasionally more than that. But usually if I’m going further than that I take the train, or fly. Or get a ride with friends or family. But I do ride a lot; for example, I’ll ride ten miles to get to the start of a group bike ride.
I’ve always commuted by bike, and gotten around by bike. And so do many other people in Silicon Valley. The majority of people in the valley have cars-- certainly not “practically everyone” though. Many do not.
BB, that main street in Mountain View is called Castro Street, and over the years it seems to have morphed into being entirely restaurants.