How to apply for a driver license if you are over 18
If you are a visitor in California over 18 and have a valid driver license from your home state or country, you may drive in this state without getting a California driver license as long as your home state license remains valid.
If you become a California resident, you must get a California driver license within 10 days. Residency is established by voting in a California election, paying resident tuition, filing for a homeowner’s property tax exemption, or any other privilege or benefit not ordinarily extended to nonresidents.