Online voter registration and pre-registration is available for eligible 16, 17, and 18 year old residents in the state of California at https://registertovote.ca.gov/
To be eligible, you must be a US Citizen and resident of California.
California youth who pre-register to vote (ages 16 and 17) will have their registration become active automatically once they turn 18 years old.
Pre-registration does not change the voting age, which is 18. Instead, it allows eligible Californians ages 16 or 17 to complete the online voter registration form providing sufficient time and opportunity to get ready to vote.
If you are registering or re-registering less than 15 days before an election you will need to complete the Same-Day Voter Registration (link to same day registartion: https://www.sos.ca.gov/elections/voter-registration/same-day-reg/) process and request your ballot in person at your county elections office or polling location. For more information on voter registration and voting locations, contact your local county elections official.
You will not need to register to vote again unless you move to a new address, change your name or change your political party preference.