Rules for notarizing minors
You can notarize the signature of a minor, however, their signature is not legally binding since they are under age. The minor still needs to be positively identified, so they need an identification document of some sort that is current, government issued, has a photo, physical description, signature, serial number, and expiration date. It is prudent to document in your journal, and on the document the age and possibly the date of birth of the signer, so everybody reading the paperwork will immediately be aware that the person is under age. Rules for notarizing minors could vary state by state, so please ask your state notary division what their recommendations are.
It would also be prudent to take a journal thumbprint of the signer unless your state discourages such an action (such as perhaps Texas and Florida).
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Identification for being notarized