@David4245
I am so sorry you are facing this issue, I can only imagine how frustrating it must be.
Airbnb won’t allow for such a rule (over 21 y/o only) as it will regarded as discriminating- I can’t lead my spaces to under 18 y/o as my insurance policy does not protect me from this and when I asked Airbnb was noted down for discriminating despite stating how restrictions prior.
I would suggest you get of instant booking, as to be able to have an open discussion with guests and set an expectations you may (we also do this if IB’ed and point out any pitfalls eg. accessible by stairs only etc and ask them to confirm they have read all listing and rule). Sorry I could not be more helpful!