Entry into Canada: Canadian law requires that all persons entering Canada carry both proof of citizenship and proof of identity. A valid U.S. passport, passport card, or NEXUS card satisfies these requirements for U.S. citizens.
Children under 16 need only present proof of U.S. citizenship.
Entry into the United States: When traveling by air from Canada, U.S. citizens are required by U.S. law to present a U.S. passport book, except as noted in the few exceptions provided on the U.S. Customs and Border Protection website.
For entry into the United States via land and sea borders, U.S. citizens must present either a U.S. passport, passport card, NEXUS card, Enhanced Drivers License, or other Western Hemisphere Travel Initiative (WHTI)-compliant document. The only exception to this requirement is for U.S. citizens under the age of 16 (or under 19, if traveling with a school, religious, or other youth group) who need only present a birth certificate, Consular Report of Birth Abroad, or naturalization certificate.