CPST here!
There is a lot of very scary and dangerous misinformation presented on this thread.
1. It doesn't matter what the California law is. If you do what is actually safe for your child, you will exceed the requirements of the law in all 50 states.
2. NO SIX YEAR OLD CHILD SHOULD RIDE WITHOUT A BOOSTER SEAT, END OF STORY.
3. No child under 4'9" should ride without a booster seat.
4. Island-poohbear, if you took your 4 year old out of a booster, you actively put his life at risk. A four year old fits just fine in a booster seat. If your child is under 4'9", he needs to be BACK in the booster seat.
5. ALWAYS ALWAYS ALWAYS use correct child restraints for your child's height and age, including in taxis and limousines.
PER NHTSA RECOMMENDATIONS THE REQUIREMENTS ARE AS FOLLOWS
Birth-twelve months: REAR FACING ONLY
Twelve months until the fourth birthday: REAR FACING UNTIL THEY OUTGROW THE SEAT. THIS MEANS AT LEAST TWO YEARS, BARE MINIMUM.
A child who is older than two and has outgrown the height and weight restrictions for rear facing may be placed in a forward facing harnessed restraint
Four years through the 8th birthday: FORWARD FACING FIVE POINT HARNESS until the child outgrows it, then a booster.
8 YEARS-TWELVE YEARS: Booster until 4'9". If your 11 year old is 4'7", he needs a booster. End of discussion. Then a lap-shoulder seatbelt.
To answer the original question: THERE IS NO SUCH THING AS A SIX YEAR OLD WHO DOESN'T NEED A BOOSTER ;-)