The minimum age of a passenger to fly an international flight without a companion must be at the age of 15. But there are exceptions to this a kid below 15 years old would be allowed to travel without a companion provided that the parent will pay an extra amount to the airline who will assign a personnel to look after the child while on board. The parent will also assure the airline that a relative or an advance party will fetch the child upon its arrival to the airport.
I'm not sure of the actual age, but for children and people with special needs, some (most) airlines will provide a child or special needs person with an attendant to watch over the person in the event they are flying alone. They will make sure they get to their destination.