From what age a minor can apply for patent under US law?

In the United States, each named inventor must execute an oath (declaration) as required by 37 Code Federal Regulations 1.63(a)(1):

(a) The inventor, or each individual who is a joint inventor of a claimed invention, in an application for patent must execute an oath or declaration directed to the application, ….

There is no minimum age for a person to sign the oath or declaration, so long as they are competent to understand what they are signing, including the claims (gasp!) and be able to appreciate the duty of disclosure to the U.S. Patent Office all information known to be material to patentability. See e.g., 37 CFR 1.63(c):

(c) A person may not execute an oath or declaration for an application unless that person has reviewed and understands the contents of the application, including the claims, and is aware of the duty to disclose to the Office all information known to the person to be material to patentability as defined in § 1.56. There is no minimum age for a person to be qualified to execute an oath or declaration, but the person must be competent to execute, i.e., understand, the document that the person is executing.

I hope this addresses your question adequately.

Best wishes for success with your invention!

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