It depends on airline that you are (or 17 years old) going to travel. In most of the airlines, the unaccompanied minor service is mandatory for children below 12 years and this service is optional for children (or adult) of 17 years. For example, in British Airways, children below the age of 12 years can’t travel alone and parents have to request for the service when making British airways reservations. If you want this service, though it is optional for you, you have to request it when purchasing British airways tickets and reservations.