My daughter flew alone when she was about 7, on a direct flight. I don't think I would have let her if she had had a connection. I was way more nervous than she was. My parents were at the airport in Florida before her plane ever took off here in CT just to ease my mind.
One thing I did for my daughter to make it easier for her was buy her a new watch and give her a goodie bag with a letter or tiny gift to unwrap every half hour on the flight.
As I recall, the airline was USAir. I felt really good about them afterwards because the attendant would not let me touch or hug my daughter after her return flight until she had checked my ID, even though my daughter greeted me with a huge "Mommy!" Maybe not all attendants do this, but it really impressed me.