@Ronald61 as per ABB standards any 18 YO may rent anyone's place and they may have as many 18YO friends over as allowed by your listing. Unless your listing states that there is some legal reason that you have an age requirement (HOA, insurance, etc). If ABB believes that you decline this request simply bc of the guest age you WILL be in violation of ToS.
My listing allows parties BUT with fees and strict requirements. So, for a 19YO my message is "Yes, of course we'd be happy to have you. We will need name, age, contact numbers for ALL guests, we will verify IDs and there will be absolutely no alcohol allowed on premises when there are any underage guests. Also, please tell me more about how many guests you're expecting and from when to when so that I can get you a quote for the party. (allowing parties does not mean they are free!!) We do not allow unregistered guests on property and there are security cameras at the entrance/parking area. I look forward to hearing from you."
This usually means that I will never hear back from them, but I'm in the clear with ABB ToS.
Also, you could consider making "party days" have a multi-day rental requirement so as to avoid the folks who want to swoop in, party it up and be gone...