I've frequently flown UM both domestically and internationally from about 9/10yrs old with stop overs at Singapore or Kuala Lumpur mostly.
So I don't get the issue. UM are seated at the back of the plane next to the crew on most airlines. They only allow other females to sit near them. We always had dads credit card (secondary card) in our name to purchase food/accommodation as needed. Tho in all my years of flying UM I've never had an issue.
But the airline could only sign us over to a guardian that was prearranged upon arrival.