In California, 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, depending on the seat’s specifications.
Source: California Highway Patrol.
In California, children under the age of 8 must be secured in a car seat or booster seat in the back seat.
For additional information, please see California car seat laws.