My dd flew to Australia one Christmas on her own at 11 but this was BA and a long time ago. She was what they called an UnMin (Unaccompanied Minor) so was 'handed over' to them, so to speak, at check in, and delivered at the other end to her friend and friends parents. I forget whether we had to pay extra for the service.
I imagine the more expensive, traditional airlines will still provide this service but can't imagine the budget ones doing so.
I used to work for an airline long ago, and sometimes we had children as young as 8 travelling as UnMins, going from e.g. Africa where parents were working, to their boarding school in the U.K. But with such young children there would be an 'auntie' provided by the airline, presumably for a fee, to look after them. It was v sad sometimes, though - the children crying and not wanting to go at all.
I've actually just remembered that quite recently a young nephew of ours, no more than 12, flew on his own as UnMin from Singapore on Sing. Airlines. Unfortunately we'd been given quite the wrong info about when to meet him at Heathrow, so there was a worried call from the airline and Dh had to shoot off, but nephew was quite unconcerned - staff had looked after him v well and taken him for a burger!