All my guest are informed there we take a photo copy of each adults IDs. That every guest must be over 18, unlessa accompanied by their parents. All our guests under 25 must pay an underage fee per adult under 25. Make it high. Then let them know they cancel for no penalty if they do not wish to pay the fees. If Airbnb says anything, tell them you updated your listing on you phone and somehow it didn’t stick. And you were “shocked that Airbnb would have a glitch that wouldn’t save your new updates” then update your listing. Or go ahead and update it, then tell the guest. And when. Airbnb checks it they will see it shows what you said. We make all large parties a d underage parties pay an additional fee. Hope that helps.