It depends, if you're going through quarantine/international you sometimes have to pick up your bags and then re-check them. For example, flying SYD to SFO via LAX.

I would never check a bag on a hidden city flight.

Flying through the US for any international flight you always have to collect your bags, clear customs and immigration, then re-check them.

The US does not have the concept of international transit lounges like other countries do, where you can get off plane 1 and sit in a room that is not technically inside the country (so you don't clear customs or immigration), then just get on plane 2 and go to another country.

I suspect there would be cases where it's cheaper to book a connecting flight through the US to some other country (or US city) and just get off at your first stop.

Makes sense, given the massive security theater in the US and the fact that it's so off the beaten path of the rest of the world that most flights probably don't just "transit" through the US on the way to somewhere else.

I was in South America for a year, and tons of Europeans had a nightmare flying to South America because the majority of flights "transited" the US, so they had to get a visa and all that for a 1 hour layover each way.

If the youth is checking in by themselves, it is not unreasonable to believe that they brought an extra bag and would need to check in something... or that they got a bag that didn't fit into the "it needs to be this small to check in".

I've... stretched the definition of "one carry on and one camera bag" in the past (my camera bag is about https://www.bhphotovideo.com/c/product/1547750-REG/lowepro_l... and then I've got a small hiking backpack with laptop pouch for my carry on bag). If it wasn't a camera bag (or my single carry on wasn't as compressible) I would likely have had to check one in at the gate... as it was I've always been able to put the camera bag under the seat in front of me and the carry in in the space between/around the harder shelled monstrosities that people try to stuff into the overhead.

If I had two carry on bags rather than a carry on and camera bag ("We may allow up to two carry on bags, space permitting in the plane") or one didn't fit in the bracket at the counter without smooshing I would would have had to check one in.

(Aside: I have never skiplagged - I just don't trust camera bags or laptops to baggage handlers and if packing light results in no checked luggage, all the better)

If the youth didn't know the nuances of how to pack for such a situation there are a number of things that would be gotchas that would indicate to the employee at the counter that this would be skiplagging... along with the existing red flags of the drivers license, and the family history of doing it.

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It would have been interesting if they (the airline) were to rebook someone they suspect of skiplagging onto a partner airline that has a different connecting stop. "This flight is full, so we're booking you on Delta with a connection in Atlanta. Fortunately, it will get into New York City an hour earlier, we will be contacting your guardian there about the earlier arrival."