D.B.
I agree about supervision. I feel that way about any sleepovers and extended visits, actually, if kids have access to computers and phones and internet.
Our town, and many surrounding towns, have chapters of Venture Crew, which is a coed leadership and adventure group for high school students. They go skiing and camping and hiking and sailing - day trips, weekends, and 1 week excursions. Men and women chaperones go along and are in the same tents and bunks. It works great.
If you and your husband or other adult chaperones of both genders are sleeping in the same room, it's fine at any age. Be sure the other parents know.
But I'd be surprised if kids want to do this past a certain age. Once kids are prepubescent, I think there are privacy issues that are hard to manage. Bathroom privacy is one, girls getting periods at the worst possible moment is another. Lots o consider.
It's worth noting that trying to prevent sexual activity is not just based on 2 genders. Gay and bisexual kids can have contact and experiment in all-boy or all-girl parties, so let's be realistic.