Each state has different requirements regarding its child safety seat or booster seat law, and in California, age and weight factor heavily into the regulations.
Children under 2 years of age shall ride in a rear-facing car seat unless the child weighs 40 or more pounds OR is 40 or more inches tall. The child shall be secured in a manner that complies with the height and weight limits specified by the manufacturer of the car seat. (California Vehicle Code Section 27360.)
Children under the age of 8 must be secured in a car seat or booster seat in the back seat.
Children who are 8 years of age OR have reached 4'9" in height may be secured by a booster seat, but at a minimum must be secured by a safety belt. (California Vehicle Code Section 27363.)
Passengers who are 16 years of age and over are subject to California's Mandatory Seat Belt law.
In regards to booster seats, the CHP says:
"A child is ready for a booster seat when they have outgrown the weight or height limit of their forward-facing harnesses, which is typically between 40 and 65 pounds. Read the forward-facing car seat’s owner’s manual to determine height and weight limits, and keep your child in a harnessed seat for as long as possible.
Children at this stage are not yet ready for adult safety belts and should use belt-positioning booster seats until they are at least 4'9" and between 8 and 12 years old. Safety belts are designed for 165-pound male adults, so it's no wonder that research shows poorly fitting adult belts can injure children."