In the absence of a passport, a birth certificate (original or a certified copy), plus a picture ID card issued by a federal, state or local government agency is required. A voter registration card or Social Security Card are not considered to be proof of citizenship. A passport (valid or expired) is highly recommended as it will speed-up your check-in formalities as well as your departure procedures with government officials. Children under 16 years of age do not require a picture ID. Visas are not required for U.S. citizens.