It should be noted that before you start the naturalization process (Form N-400) you need to check whether you are eligible to become a US citizen. There is a list of certain requirements which need to be met. Age is one of them, a person should be 18 years old to be able to file the form.
Application for Certificate of Citizenship (Form N-600) or Application for Citizenship and Issuance of a Certificate under Section 322 (Form N-600K) is applied to children younger than 18 years old who are about to become citizens.
October 20, 2019