The twelve and up age range seems reasonable to me. Most twelve year olds are old enough to behave iin public, listen to the program director, and eat nicely in the dining room. (I am thinking of the European river cruises here). If a teenager didn't want to attend every lecture, class, etc., he or she could always watch the scenery or find other activities. I don't think the program would necessarily have to be adapted for that middle school age range, and it certainly wouldn't need any adjusting for the high school or college age group.
Ideally you would want one adult for each child attending, but there might be exceptions to that...a grandmother taking two grandchildren, for instance. Some individual attention to room assignments would probably be necessary during the booking phase, so that the rooms of a party would be across from or next to one another. And the adults of the party would have to understand from the beginning that they alone would be responsible for the "care and feeding" of their young companions.......not fair to put that on the program director.
We have taken a ten year old to Egypt successfully, kids eight to sixteen on an African safari, kids twelve to twenty to South America, and kids eight and up to Europe. Great experiences, and wonderful for their education! Does anyone else have younger family members who might be invited along for a trip like this?