You have to be at 18 years of age or older or an emancipated adult to create a will that is valid in California, and you must be of sound mind.
The will has to be signed in front of two witnesses, and they have to sign it when they are together. This can be immediately after the testator has signed the will, or they can sign it together when they are witnessing the testator acknowledge the will.
There is no notarization requirement, and a handwritten or holographic will can be valid in the state of California (but they are not recommended).