Laws vary based on the state and the purpose of the signature, but, in most cases, notaries public can witness any signatures except their own. When a notary public is not available, witnesses generally must be at least 18 years of age and of competent mind.
Witnesses may be needed for the signing of medical consent forms or legal papers such as divorce decrees. When a witness other than a notary public is used, witnesses will need to provide some form of identification to prove that they are of legal age and confirm their identity. A notary public will have an official stamp and ID number to use.