SHould this 20 year old file N600 or the N400 citizenship application? this is about a 20 yrs old girl, she has GC dated Since 2007 and her mother became naturalized US citizen in mid 2011. her mother was asylee. and the girl came to US in 2006 as a derivative asylee. so bit confusing here is , should the 20 yr old child file N600 or the N400 if she wants to apply for US citizenship? the mother has another child aged 12 and she just applied for N600 based on her mother and was easy and became citizen last month. i know the general rule is: People 18 or older use form N400 to apply for citizenship. People who acquire citizenship from their parent/parents while under 18 years of age use form N600.