I guess it's different in other places.
When I was 16 I took an airplane to attend a UN conference in another city - alone. Okay, my accommodations were provided for, but I wasn't supervised in any other way. It was a terrific experience, being a youth delegate.
When I was 17 I joined the Canadian Army Reserve and lived in barracks for months. I could legally drink in Quebec. I worked hard, and when I wasn't working I was free to do whatever I liked in town. I had my own paycheck - paid in a huge wad of cash at the end of the month! FWIW, I did not drink to excess, have sex or run wild. I was sensible and cautious, and kept my cash in my boots until I could get to a bank.
I rented my own apartment when I was a month past my 18th birthday.
I have faith in my children's good sense - they're even more conservative than me! If I knew their friends (and knew they weren't idiots) and at least one was old enough to legally rent a hotel room, then I would certainly allow my children to travel without me at 17 or 18 years old.
That said - I would expect a call every night! I'd also be running down their legal responsibilities and going over how to handle worse-case scenarios with them before they left.
But 18 is adult, and 17 is nearly there. I don't see any reason to treat them like they're incompetent.
Of course, all this is ignoring the fact that they'd have a darn hard time getting out the door without me... "I wanna come, too! Please! I love Orlando!"