There is no law that specifies a minimum age to operate an ATV. However, all operators must be able to reach and safely operate the ATV. Parents of children under age 14 can be held legally responsible if they use an inappropriately-sized ATV. Operators ages 14-17 must have an ATV Safety Certificate or be supervised by an adult with an ATV Safety Certificate. Children 13 and under must be directly supervised by a parent or guardian at all times. You can learn more about obtaining a safety license on the California State Parks website.
Check out these Yuba City, CA ATV/OHV resources: