Unaccompanied minors?

They can.

Yeah, or a lot of people in this thread have committed crime! 

My youngest daughter, who is 11 years old, has been traveling as an UM, twice a year, for three years now. Her flight is a direct flight from Atlanta to Gatwick. Her father sees her on the plane in Atlanta and she is personally escorted to us at this end. I bring my passport to show she is my daughter and have to sign a release for her.

On her flight home, we give and the airline takes/holds all documentation on her, they put her aboard well before the flight, and then we wait until the flight is actually airborne before leaving Gatwick. She is personally escorted through immigration, etc. and is then escorted by (usually by the senior flight attendent) in Atlanta to her father. The first time she flew, she was a bit nervous; however, now she's become quite a pro. In fact, the one time I did fly with her, she resented the fact that she actually had to stand in line with me instead of getting the royal treatment.

She says the air attendents have always been very very nice to her on all her flights. And I have to say that we have always had very good experiences with Gatwick's escorts.

I believe that each airline has their own variations of what ages can do different things like change flights, fly international etc.

While I feel comfortable letting mine fly direct, I would never put them on a flight where they had to change planes or make a stopover. I also wait until the flight actually takes off before I leave the airport which is what they tell you to do. The BA lady told me they get several children a year who are stuck by flight delays and their parent has already left.

I went ballistic at their dad this year who had scheduled his own return flight from an unannounced holiday into the same airport the day the girls were due to arrive. What if his own plane had been delayed??

I believe that each airline has their own variations of what ages can do different things like change flights, fly international etc.

Hmmmm... I don't know of any airline that doesn't allow UMs to fly internationally or change flights.  They all have very similar policies in this area.  Though I am not certain, I suspect they are governed by international laws or at least international agreements.