Any passenger over the age of 17 is considered an adult and should accompany another child or infant. In other words, if a child is 0 to 3 years old, there must be another passenger accompanying them who is an adult. However, for children from 5 to 11 years, many airlines offer unaccompanied minor travel services, which they can opt for.
Also, the airline does not give a seat to infants, and they must travel on the lap of their accompanying adult.