Flight requirements can differ per airline as certain airlines categorize children differently. For instance, United Airlines considers infants to be 0 to 4 years old and children to be aged 5 to 15, which is not the same universally among all airlines.

United Airlines has specific age tiers that determine how a young traveler is classified and what services they require. For ticketing purposes, a "child" is typically an individual between the ages of 2 and 11 years old. Children in this age bracket must have their own seat and are charged the standard adult fare on most domestic and international routes, though some international destinations may offer a small discount. Infants under the age of 2 can fly as "lap children" (usually for free domestically or 10% of the adult fare internationally). For safety and supervision, United defines "Unaccompanied Minors" as children ages 5 through 14 who are traveling without an adult. For these travelers, United's mandatory unaccompanied minor service ensures they are escorted by staff through the airport and onto the plane. Once a traveler reaches the age of 15, United considers them an adult for most travel purposes, meaning they can fly alone without the mandatory supervision service, although parents can still opt for it until the age of 17 for an additional fee.