SF has plenty of young professionals who are living with roommates. I would say a lot of that is driven by the cost of housing, but also because many young people like the community that comes with roommates.

That said, I think there is a distinct inflection point in people's lives here where they either partner up or just get tired of not having a space that is entirely their own. The 2nd seems to happen to 30-35 year olds a lot.