Quickly find the most current demographic information for a specific city, county, or even zip code anywhere in the U.S. using the Community Facts guided search. Generally 5 years of data is immediately available.
A wide variety of California statistics, collected by state & local sources can be found at http://data.ca.gov/
A tremendous amount of data about the U.S., current and historical, is available through the U.S. Census. The best guide I've seen to using that data is from a librarian at UC Berkeley: http://guides.lib.berkeley.edu/USCensus/Intro