For adopted child's Naturalization file Form N-400 if he/she is over 18years age

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Once your adopted child qualifies as your child for immigration purposes, you can file an I-130 relative petition. Your adopted child will automatically become a U.S. citizen if the child becomes a permanent resident and the adoption is completed and a Form N-600 is filed before the child reaches 16 years of age. If your adopted child turns 18 years old before he or she becomes a permanent resident, then your child may apply for naturalization using Form N-400, Application for Naturalization, after being a permanent resident for 5 years.