Effective January 1st, 2006, the new law mandates that all teen drivers must have their license for one year before they can drive between the hours of 11:00 P.M. through 5 A.M. or have young passengers without an adult present in the vehicle.
The development of the law was based on studies of teen driving patterns and safety concerns. "The number of crashes involving California's 16-year-old drivers between 11 p.m. and midnight is nearly 13 percent higher than we would expect when we take into account the amount of driving they do at that hour," said Steven Bloch, Ph.D., the Auto Club's senior research associate who analyzed the California teen crash data.
Note that teens who obtain their license before the 1st still are under the law’s auspices if their license is less than a year old.
Further details can be found here, here and here.