Orange and demons - I'm a retired teacher and I've known lots of 16-year-olds too. And as I said upthread, we left our younger son, not for two weeks but for two years, while he did his A levels. (Because we were teaching in Lesotho and he didn't want to do the IB - which is what was on offer locally - so returned to England for A levels.) I don't have a problem with 16-year-olds being left alone - on the contrary, it's good for them.
MajesticWhine - do you seriously consider young people of this age to be "just kids"? Are you proposing to throw a switch when they reach 18 and have them suddenly turn into adults? Parenting teens is all about gradually letting go of the reins, and by 16 they should be making their own decisions with parents just giving advice. In Scotland, that's the law. In England we are a bit more flexible.
wishiwasntme - although (because?) our son lived by himself when he was 16, doesn't mean he gave up on family life. We get on famously now (and did then, when we visited). Just because a teenager doesn't want to go on a family holiday, doesn't mean they reject family life completely.
And to all those who would "insist" that they go on holiday with you - how, exactly? Would you drag them, kicking and screaming, into the car/taxi/train/whatever? If they've told you they don't want to go - and have given reasonable notice - then don't be surprised to come downstairs on the morning of the holiday to find they're not there. If you're lucky they may leave you a note. What will you do then? Go on your holiday and worry about them or cancel and wait for them to come back? You really need to resolve this in advance and just "insisting" isn't going to work. An adult discussion might. Ask them to do a risk assessment and come up with a contingency plan for all the things that might go wrong. They will be better prepared and it might even put your mind at rest as well.