It can depend on the airline but the minimum age for most airlines to allow a child to fly alone without a parent or guardian is 12 years of age.
The child must be 12 years of age before they'll be allowed to fly alone without a parent or guardian.
It may be as old as 13 years old for some airlines because it can all depend on their policies that they have in place.
But if the child is at least 12 years old then you'll likely be allowed to have the child fly alone.