California law as of January 1, 2005, states that children must be secured in the back seat of a vehicle until they are either 6 years old OR 60 pounds (with limited exceptions). Further, a child may never (without exception) ride in the front seat of a motor vehicle with an active passenger air bag if the child is (1) under one year of age and weighs less than 20 pounds or (2) riding in a rear-facing child passenger restraint system.
Keep in mind, however, that these are the minimum standards established by the state. The guidelines outlined in the owner's manual of your specific seat or your vehicle will be more stringent and should be followed. For example, studies have shown that it is safer for children to remain in a rear-facing car seat until they are both 1 year-old AND 20 pounds. Even though this is not a legal requirement, it is sound advice that should be followed.
Also, children may not ride in the back of a pickup or flatbed truck (covered or uncovered) unless secured to a restraint system that meets federal standards. And finally, no child under 6 years old may be left unattended in a car (meaning no one over 12 years old is present) if (a) the health or safety of the child is at risk, (b) the engine is running, or (c) the keys are in the ignition.