A Passport
The best form of identification for traveling is the passport. This internationally recognized travel document is official proof of both identity and nationality and is accepted at all border crossings around the globe, whether by land, sea or air.
Alternatively, there is a US Passport Card available now that costs less than a passport and can fit in a wallet, but allows limited use in that only land and sea border crossings will accept it.
Other Forms of Identification
Currently, several US states and Canadian provinces are issuing an Enhanced Driver's License (EDL) that denotes both identity and nationality, making it suitable for border crossing. Like the US Passport Card, it is only acceptable at land and sea border crossings.
A few Trusted Traveler Programs have emerged that issue enrollment cards to low-risk travelers. The FAST, NEXUS and SENTRI cards are accepted at land and sea borders. NEXUS cards are also approved for air borders at airports with NEXUS kiosks.
Youth Requirements
In addition to the forms of identification acceptable for adults, children under 16 may present a birth certificate (original or copy), a Consular Report of Birth Abroad, a Canadian Citizenship Card or a Naturalization Certificate at land and sea border crossings.
Children under 19 and traveling in a group (school, church, sports team, etc.) may also present these forms of identification. The group may need to present a letter from the organization stating the names and contact information of all children in the group and their adult chaperone.