Just to clarify a few things;
-No U.S. company will allow a child under age 5 on board without an adult so flying alone under that age is a non-issue.
-Yes, the vast majority of the time, 5 year olds are with older siblings.
-Flight Attendants don't "babysit" UM's. We did watch over them but they had to be able to handle themselves with the toilet, etc. alone. For hygiene reasons, we could not assist with that. Logically, any child who can't handle these tasks should not be sent as a UM, including if the child has any behavior issues.
-Once on the ground, the UM children are handed over to a ground agent. The Flight Attendants don't work on the ground and don't stay with them. That article where the woman asks "Where were the attendants?" was just plain wrong. We were careful present the child to the ground agent with the paperwork before disembarking. Then the ground agent hands them over to the waiting adult, named on the paperwork. They're supposed to check ID's, even if the child is calling out "grandma!"
At least with my airlines, if this wasn't followed, whomever was responsible was in big, big trouble! (Sort of happened once. The kid was trying to escape a court-ordered visit but since it was international, he didn't get far. We all got big letters in our mailboxes warning us of the importance...)
-Not all airports give "gate-passes" and honestly, this should not be a deal-breaker. If you're in some tiny airport, you're more likely to get one. But if not, you will not be parted from your child until the airline employee is there and ready to take him or her. That person, by the way, has no other duties until the child(ren) are on board. The UM's are also always introduced to the Chief Purser (or "First Flight Attendant" on some airlines).
-Be careful to use the right terms. "Direct" means the plane has the same flight number. It CAN change aircraft. A "non-stop" doesn't stop at all. There are "direct" flights which stop but some passengers get off and other stay on. Be sure of what you're booking your child on. Usually 5 year olds are only on non-stop. They wont be allowed on direct flights with an aircraft change.
-Teenagers are fine traveling without any oversight but be aware that they often aren't allowed to book hotels on their own. If your offspring has a delay or cancellation, he or she might find themselves in the airport overnight. Obviously, this is not a problem on a non-stop where there are friends and/or relatives on both ends but keep this in mind if they're connecting...