He was probably plugged to his phone listening to some garbage or playing a video game.
Yeah, I could see that as a factor in the young man getting on the wrong flight.
Another factor might be the use of a single, unsegregated, large seating areas at a single gate. I’ve been in a number of regional and major hub airports where passengers were waiting for three or four different flights at the same gate. The only way they were controlled was by each traveler paying close attention to flight announcements by the gate agents as to which flight was being called forward for boarding. Compounding this is the sometimes incomplete announcements as different seating sections inside the cabin are called to board.
So if you’ve got a kid who’s already zoned out on his earbuds, he might’ve just noted people around him starting to get up and assumed it was time to board. - especially if he’s not used to traveling by airlines and with their multiplex use of boarding gates.
Frontier Airlines policy of allowing 15-year-old and older minors to fly as unaccompanied adults has as an unstated assumption that, at 15, they are old enough and smart enough to function in a controlled adult environment. Obviously, this introduces the opportunity to find out that some are not.
For information, it might be interesting to know how many adults end up in the wrong location for the same reasons; inexperienced, confused, or not paying attention to the announcements.