No, a 15-year-old U.S. citizen cannot travel to Mexico without a valid passport if they are traveling by air. According to 2026 Western Hemisphere Travel Initiative (WHTI) regulations, all travelers—including infants and minors—must present a Passport Book for international air travel. If the 15-year-old is traveling by land or sea, they are still required to have a valid travel document, which could be a Passport Card or a Passport Book. While some "Closed-Loop" cruises (trips that start and end at the same U.S. port) may technically allow minors under 16 to use a certified birth certificate and a government photo ID for re-entry into the U.S., the Mexican government still requires a passport for legal entry into their country. Additionally, if a minor is traveling with only one parent or another adult, Mexico strongly recommends carrying a notarized consent letter from the non-traveling parent(s) to prevent issues with immigration authorities regarding child custody laws.