Like all other states, California implements seat belt laws for the protection of its citizens. With older cars, these laws can be murky because rules and standards in car manufacturing have changed over the years.
The seat-belt law in California stipulates that a car adhere only to seat-belt standards that were in place at the time the car was manufactured. Therefore, a classic or antique car built before 1964 -- the year in which seat belts became a mandatory standard in all cars -- is exempt from this law.
While these older cars are not required to be fitted with newer seat belts, it is advisable to have them installed if this is a car that you drive regularly. Additionally, California law does not allow a driver or their passengers to drive on the highway without wearing functional seat belts.
In California, it is illegal to have a child age 6 and under ride in a vehicle without being properly secured in a car or booster seat in the backseat. If you plan to have a child in this age range ride in your vintage car, you must have seat belts to accommodate them.