Anyone parent can sign the child's application. Note that the child's age should be between 0-17. If the child is 18 years or older, they should sign themselves on the SSN update application form. If a child's age is between 17 and 18, it is considered 17 until the child turns 18. Example: If you are a child on an H4 visa aged 17 years and 2 months who has just got the green card approval, your parent can sign on your behalf on the form. If you are a child 19 years old, you should sign the form yourself.