She is your daughter and you know best, but to let a 16yr old go on holiday without an adult is surely asking for trouble, theres the booze and the sex for starters and though she might be quite grown up and sensible she is no match for a 21yr old foreign romeo.
I dont think a tour company will accept their booking anyway, who will be responsible for them in case of an accident or god forbid rape ?. pls think again and wait till she is 18and old enough to take resposibility for herself.