Travellers under the age of 18 are required to carry a government-issued photo ID according to the United Airlines Id Requirements For Minors. A photo ID, such as a driver's licence, passport, or state-issued ID card, must always be carried by passengers. Additional kinds of identification accepted by the airline include military ID cards, school IDs, and student IDs.
https://airlinespolicy.com/una....ccompanied-minors/un