The minimum age for a child to travel alone is 5 years of age and only when it is duly assisted and has to pay a supplement per way (rates click here)
Assistance is compulsory for Unaccompanied Minors (UM), i.e. children from 5 up to 11 years of age who are not accompanied by a person who is at least 18 years old.
For "Young Passengers"(YP), i.e. youngsters aged 12 up to 18, assistance is optional and can be applied for.

If the assistance is not applied for in advance, this surcharge will be increased by an administrative fee of € 7.50 per leg of the journey and per child at the airport.
The regulations above only apply for TUI fly or Freebird (destination Turkey) flights. For other airline companies 'special requests' always require confirmation and surcharges are different.

Just like adults, children must be in possession of their own travel papers. Please refer to the travel formalities.

It is also strongly advised to have the municipal authority of your place of residence draw up a document in which the child's parents / guardians declare that the child is allowed to travel alone. The minor needs to bring this document on each flight.
Attention: For Portugal, the travel authorisation for minors is mandatory!