OK, I stand corrected. I did some checking around, and I've found that some US domestic airlines have recently raised the "unaccompanied minor" upper limit to 14. (It was universally 12 as recently as 4 years ago, when my niece was flying to see us on a regular basis; she's 17 now.) Among them are NWA, CO, USAir, Delta, and as of just a few months ago, Frontier. BTW, the lower age limit for a child to fly as an unaccompanied minor in the US is age 5, that limit is set by the FAA.
Per their current website info, as of 12/8/04, United and AA, O'Hare's two largest carriers, do not require payment of the unaccompanied minor fee for anyone age 14. SWA does not consider a 14 yo an unaccompanied minor, either. ATA does consider a 14 yo an unaccompanied minor, but allows them to travel without the service if they are accompanied by someone who is at least 15.
14 yos are not young enough to be eligible for a child's fare, and are also not required to show ID. If the young person in question claims to be 15, I don't see that there is anything that the airline can really do to enforce payment of the add'l fee, or to impose the itinerary restrictions that apply to UM passengers. There would be no recourse for the passenger if he/she experienced an overnight delay, but that has been a problem with younger travelers for a long time; most hotels will not accept checkins from parties where the oldest guest is not at least 18, and sometimes 21. The way around it (other than sleeping on the airport floor) is usually for a parent to call ahead and pre-pay the hotel charges with a credit card, including a damage deposit.
I'm not advocating "cheating" on the fee, but I can understand why someone might do it if a restricted itinerary was booked before the buyer realized that the UM fee might apply. Changes could get quite expensive, at least in this case, where 2 tickets might have to be changed.