I would not bother with UM for a 14 YOA on a non-stop, but I most definitely would with a connection.

I say this as someone who flew alone at 14 and made connections. But, that was a long time ago and times have changed.

The UM fee is worth the expense on connections not because a 14 YOA can't walk between gates, but because the carrier will take care of the kid in IRROPS. This may include paying two employees to spend the night if a hotel is required (14 YOA can't rent a room).

An alternative here is if the 14 YOA is connecting at a city where someone has an emergency backup, e.g., a reliable relative or friend who could, in the case of IRROPS, pick the kid up and return him to the airport, I would again forget the UM fee. It's unlikely to be needed, but you want it in place just in case.