my son has flown 3 times alone, london-chicago with BA. he was 14 the first time and i didn't pay the extra fee to have someone track him.(his choice). of course this limits him to direct flights but he is always highly motivated, so he never complains about any of it. i was told by BA to have him carry a letter, giving permission for him to travel alone and i've also added important phone numbers both here and in the states. once he did have trouble coming through immigration here because we'd moved and he couldn't remember his new address and got nervous and forgot my mobile number........
on the positive side, he got upgraded to first class when he flew home in june!!! he said he was all settled in his seat when they called his name overhead to make himself known to a flight attendent, he thought he was in trouble but they said let me show you to your new seat...in first class!!!! not sure if they were expecting a young business man in an armani suit but they got a scruffy 15 year old in cargo shorts, t-shirt, sneakers shuffling along with his backpack!! he loved it!!
in general, i think most kids who are fairly independent, can cope by 14 years old, and i see it as a huge learning experience!!