I lived on roughly the same salary the first year I lived in SF. I was 24 at the time, and it was fine because I was working non-stop and didn't have much of a life, but I wouldn't want to do it again for long periods of time.
I'm 24 and don't work most of the time, hence the lower salary, and I still manage to go out a lot and enjoy the city.
There's just so much free or cheap entertainment here, you don't need a car, and it's usually sunny enough that if you get bored, just walking around is a pretty viable option. Living here doesn't have to be expensive, people just make it that way.
I lived on roughly the same salary the first year I lived in SF. I was 24 at the time, and it was fine because I was working non-stop and didn't have much of a life, but I wouldn't want to do it again for long periods of time.