Within the United States, a child under the age of 18 typically does not need a passport for domestic air travel, nor do they usually require any form of government-issued photo ID. TSA regulations state that travelers under 18 do not need to provide identification when traveling with a companion within the U.S. as long as the adult has valid ID. However, the airline itself may have its own policies, especially for verifying the age of a "lap child" (under 2) or for a minor traveling as an "Unaccompanied Minor." In these cases, a birth certificate (or a digital copy) is the most common and recommended document to carry as proof of age. In 2026, as the REAL ID Act is fully implemented, the rules for adults have become stricter, but children remain exempt from the photo ID requirement for domestic flights. It is important to note that if the domestic flight involves a connection that passes through an international terminal or if the destination is a U.S. territory like Guam or the U.S. Virgin Islands, carrying a birth certificate or passport is highly advisable to avoid any potential confusion at security or boarding.