Once they turn 18, teens are moved into the custody of the Department of Homeland Security – more specifically, US Immigration and Customs Enforcement, a law enforcement agency known as ICE. Migrant youth cannot, by law, stay in the shelters that housed them before they turned 18.

When a traveler turns 18, they are legally considered an adult in the eyes of the airline and the "Unaccompanied Minor" (UM) status ceases immediately. They are no longer eligible for, nor required to use, the airline's mandatory supervision services, which typically apply to children ages 5–14 (and are optional for ages 15–17). Upon reaching 18, the traveler can book flights, check in, and navigate airports entirely on their own without needing a parent or guardian to sign release forms or obtain gate passes. Any UM fees previously paid for future travel would technically no longer be applicable, though you would need to contact the airline to update the passenger's age and status in the system. It is important to note that while they are adults for travel purposes, some hotels still require guests to be 21 to check in, so while the flight portion becomes simpler, the accommodation logistics for a new 18-year-old traveler still require careful planning.