Before you begin the naturalization process, you’ll need to check your eligibility to become a U.S. citizen. An applicant must meet certain requirements. And one of requirements is age, you need to be at least 18 years old to file Form N-400.
If you want to apply for citizenship for a child who is under 18 years old, you should use the Application for Certificate of Citizenship (Form N-600) or Application for Citizenship and Issuance of a Certificate under Section 322 (Form N-600K).