It looks like CA regulations do not allow admitting a child to public K before the 5th birthday, although districts may, at their discretion, admit a late birthday to K on or after the 5th birthday, even if it's in the middle of the year. If an early-admit student finishes kindergarten in public or private, the next school year still has to begin in K, for at least a trial period, after which, again at the district's discretion, they may move the child up to first grade. The CDE guidelines suggest social maturity, physical development, and 95th %ile cognition as criteria for local districts to consider.
http://www.cde.ca.gov/ci/gs/em/kinderinfo.aspIn short, it appears that it would be a violation of CA education regs to admit a child younger than 5 to public K. Doing so would endanger apportionment to the district (aka, funding). (TK is a different story, but only applies to birthdays from Sep 2 to Dec 2, and still shunts them into another year of K.)
