Hi Hayley
My understanding is that it is the guest who books who needs to be 18, unless the host asks you for the ages of all the parties coming along they are unlikely to know.
If you like and they query it, you can say all guests will be 18 at the time of travel.
You are right some hosts are worried about hosting groups of young people because of partying and generally not looking after the place.
Just write a friendly note with your booking explaining you are there to experience the culture and food and talk about your plans. Confirm that you are a clean, tidy group of young women and will look after the place as you would your own home. (although teenagers aren't known for keeping their own place tidy so maybe scrub the last bit 🙂 )
It sounds like a wonderful experience for you all. Good luck with your trip.