Laws change over time, and as lawful firearm owners it's our responsibility to keep current on them. California laws regarding firearm loans changed starting in 2017, so it seems like a good time to recap what you can and can not legally do.
In general when you let someone else use your firearms, you are loaning it to them. However there is a legal difference between letting someone use your firearm in your presence versus letting them borrow it for several days. For the purposes of this article we'll refer to the latter (borrowing it) as loaning.
California Penal Code 27545 allows infrequent loans of firearms (rifle, shotgun or handgun) to any of the following: spouse, registered domestic partner, parent, child, sibling, grandparent or grandchild. Furthermore the loan must be for a lawful purpose, can not exceed 30 days, and the recipient must have a Firearm Safety Certificate. If the firearm being loaned is a handgun, the handgun must be registered to the person loaning it.
You can let anyone eighteen or older use your firearm (rifle, shotgun or handgun) at a target shooting facility while they are in your presence. The person using your firearm must not be prohibited from possessing firearms under state or federal law. (California Penal Code 27885) This also applies to lawfully registered "assault weapons".
You can loan a rifle or shotgun to a licensed hunter as long as the loan does not exceed the duration of the hunting season for which the firearm is to be used. This does not apply to handguns. (California Penal Code 27950)
You can loan a firearm (rifle, shotgun or handgun) and ammunition to a minor if it's used for target practice, hunting or a firearm training course. The minor must posesses written consent from their parent or legal guardian, who is not a prohibited person, giving permission for the minor to posesss the firearm and ammunition.