It all depends on the airline, but generally for Scheduled flights:

Children under 5 years old cannot travel alone; they must always be accompanied by an adult.

Children from 5 to 12 years old can travel alone, but will always need to request the accompanied minor service, subject to availability.

Most airlines allow children from 12 to 15 years old to travel alone but must also request the accompanied minor service.

When traveling with a low cost airline, children generally must be 16 years old or over to be able to travel alone, otherwise boarding will be denied.

We always recommend that you check with the specific airline as this may vary from one airline to another.