I had a 17 year old fly to France for a month of walking and biking - he was first of three friends to arrive - so arrived by himself, transferred, found the place they had hired etc. He did three weeks of 'play' and 10 days of work on a cycling event before flying home. It was a place we had been as a family before.
His brother then flew at 16.9(!) years old to meet him a couple of summers later in Switzerland - as the eldest at now 19 was driving around Europe in his own converted van with bike in for 8 weeks+. They travelled together and had a great time.
Mine are sensible lads, really focussed on bikes and mountains, and even though they like a beer and a few nights out seem really mature about things. And yes I do know some gory details of nights out I need to not think about - but I also know that they spend weeks riding mountains and living life to the full.
One is about to graduate and head to Canada for a few years work and riding, one currently in Australia on a gap year, having got on a plane the week after his 18th birthday.
Age to me is not the issue - their character and sense of judgement, 'streetwise' if you will, and the purpose of their visit is more the issue. Planes and airports are safe. When you leave the airport, that is when they need their wits about them.