Any passenger age 12 or older is booked as an "adult" on all web sites and airlines due to the airlines' pricing guidelines. However, any child under age 18 is legally "a minor," so the airlines have special rules for minors who travel without an adult present. See as follows:

Unaccompanied Minors:
You must contact the airline directly before purchasing a ticket for an unaccompanied minor. Most airlines allow unaccompanied minors to travel on their flights, but only with restrictions. Age limits for each restriction vary by airline. Generally, children under age 5 may never travel alone. Most airlines require special escort services for children ages 5-14, and some require it through age 17. Additional fees apply (usually $25-100 each way) and are paid at check-in. Most airlines do not allow unaccompanied minors to travel on the last connecting flight of the day. Other carriers require unaccompanied minors to travel on non-stop flights only. The specific connections required vary by airline, so contact the airlines prior to making a reservation.

Infants (children under 2):
If you are traveling with an infant (a child less than 24 months old) in lap, there is usually no charge for US travel. However, the airline may charge a special fare for International travel. Also, for any infant with a seat all to himself or herself, the regular child fare will be paid (not infant rate). Please check with the airline(s) for any documentation that may be required for boarding (such as a birth certificate).