Yes, as the parent of a 14 year-old currently attending boarding school in Europe, the way that many airlines have in recent years increased their minimum age rules to fly unaccompanied creates huge problems and costs. We've had to become experts in code share logistics because, for example, Swiss Air will happily accept 13 and above but they code share flights with Lufthansa who requires 15. Whether your kid can board the flight depends on which airline's code is used for the flight number, and it's not always clear on the airline's own websites (much less other sites), despite being the same seat on the same plane.
The real mess happens when a flight is cancelled and the airline rebooks passengers already in-route. Last year our kid was on an Iceland Air route in a connecting city when they cancelled the next leg. Someone at their flight operations center "helpfully" rebooked the ticket to a British Air flight leaving that city an hour later, except BA has a 14 limit and denied boarding (kid was a few weeks shy of 14 at the time). In fact, the BA gate agents couldn't even understand how it was possible for a 13 year-old to be issued a ticket (because it was done in the back-end inter-airline system). So our kid ends up stranded in a distant connecting city. We ultimately had to buy a last-minute one-way ticket on a third airline to a different city for a connection on an airline that would board her. It took months of calls to eventually get a refund from Iceland Air for a multi-thousand dollar business class ticket, on a flight they cancelled. (note: For anyone concerned, our kid is a hyper-savvy frequent flyer who grew up flying international routes. Also, for international routes we always book her business class in an isolated seat that's in its own row right next to the crew galley.)
We have a friend who's kid goes to a boarding school in the U.S. (but on the opposite coast). All the major U.S airlines now have 15 or 16 age limits. For spring break a few weeks ago, they had no choice but for one parent to fly across the country and back, both ways, just to "accompany" their kid past the gate boarding agents. Because almost all U.S airlines are now unaccompanied kid hostile, our kid can't even connect internationally from our local airport. Instead we have to drive her three hours to an airport that international airlines fly direct from.