20 C.F.R. § 408.315 – Who may sign your application?

(a) When you must sign. If you are mentally competent, and physically able to do so, you must sign your own application.
(b) When someone else may sign for you.
1. (1) If you are mentally incompetent, or physically unable to sign, your application may be signed by a court-appointed representative or a person who is responsible for your care, including a relative. If you are in the care of an institution, the manager or principal officer of the institution may sign your application.
2. (2) If it is necessary to protect you from losing benefits and there is good cause why you could not sign the application, we may accept an application signed by someone other than you or a person described in paragraph (b)(1) of this section.
  Example:Mr. Smith comes to a Social Security office a few days before the end of a month to file an application for SVB for his neighbor, Mr. Jones. Mr. Jones, a 68-year-old widower, just suffered a heart attack and is in the hospital. He asked Mr. Smith to file the application for him. We will accept an application signed by Mr. Smith because it would not be possible to have Mr. Jones sign and file the application until the next calendar month and a loss of one month's benefits would result.