Maybe the teen turns 15 during the trip? You are technically supposed to book the child at the age on the return flight. Maybe the mother also booked him as an adult.

ETA: on SAS's website, I just tried booking a single youth ticket (ages 12-25), and it only allows me to book from one of their US gateways, not from places like DEN or RDU.

While that is true, most major OTAs I have tried won't allow you to book anyone 15 and under as a single ticket.

Glad the kid arrived safely at the destination, when all is said and done. The airlines involved here should definitely review their UM policies at a high corporate level and make immediate changes.

One thing I want to highlight - the brother's sister commented on the View From The Wing blog,
If that's true, then he should not be flying solo in the way he did. There are so many things that could go wrong (the thing that did go wrong being one example) that a non-outspoken 14-year-old should not be flying alone on a connecting international itinerary, period. There is no shame in not being outspoken; in many cases it's a virtue, as our society has too many aggressively outspoken people. But I digress.

One could argue the mother was irresponsible putting her son in this situation. Of course nothing here is the mother's fault; I am only speaking of the mother's responsibility. She may be very upset to read these words. However we all have the same goal: we want her kid (and all kids) to be transported safely and correctly. While we wish all airlines were perfectly competent in all services provided (especially those forced on you), the best way to ensure the ultimate goal may be to personally accompany her kid.

While UA did screw up, SAS is the party that ACTUALLY PUT HIM ON THE WRONG PLANE.

It depends on the stakes. I imagine most Americans won't ask to return to the gate to pick up shaving cream they left behind in the airport lounge. But to ask to return to the gate because they are on the wrong flight? I'd definitely do that. As for the candles and flowers, the border agent at the incorrect international destination won't bear those either....

UA messed up no doubt.

I really don't know why Americans are so up-tight when it comes to UM. I did my first solo international trip when I was 13 and we were always very careful to avoid any of US based airlines.

Many parents in Asia send their 13/14/15 year old kids unaccompanied to the US or Europe all the time.

That's a long sentence to say "I don't really know, I'm assuming...." because there is no process for UA to provide a UM service that is far outside their CoC, to the instance where a) the UM agent isn't delivering a UM to the plane, but retreiving one, and b) moving from Terminal C to Terminal B, and c) have that process established and executed on the same day?

Sorry, I did get the airlines mixed up. It was Eurowings that actually put him on the wrong plane. An actual check of the boarding pass would have prevented this (quite frankly either by UA or Eurowings)

Quite honestly, I don't know who messed up. We don't even know whether this was a single ticket or two tickets. If it was two tickets, maybe the poor RDU agent didn't even know about the connecting flight and it was the kid that brought this up to the EWR walker. Something of the sort, I shouldn't be going to baggage claim, I should be going to my flight to Europe. Maybe the the SK and the EW flight were boarding at contiguous gates?

As a person who works for a large organization that offers complex products, I can tell you that large organizations with complex products only work well when all of the rules are followed. You can't spitball things and hope that they will work. Failing to follow rules should have consequences for whoever messed up. I hope the FTers listening who ask for rules to be broken to allow them to get something that they desire read this thread and see how wrong that can go.